diff --git a/src/libs/clangbackendipc/sourcerangesanddiagnosticsforquerymessage.h b/src/libs/clangbackendipc/sourcerangesanddiagnosticsforquerymessage.h
index f6c17a95290..6634d501956 100644
--- a/src/libs/clangbackendipc/sourcerangesanddiagnosticsforquerymessage.h
+++ b/src/libs/clangbackendipc/sourcerangesanddiagnosticsforquerymessage.h
@@ -47,6 +47,11 @@ public:
         return sourceRangesContainer;
     }
 
+    SourceRangesContainer &sourceRanges()
+    {
+        return sourceRangesContainer;
+    }
+
     const std::vector<DynamicASTMatcherDiagnosticContainer> &diagnostics() const
     {
         return diagnosticContainers;
diff --git a/src/libs/clangbackendipc/sourcerangescontainer.h b/src/libs/clangbackendipc/sourcerangescontainer.h
index 11ea542136a..147b7cb53ae 100644
--- a/src/libs/clangbackendipc/sourcerangescontainer.h
+++ b/src/libs/clangbackendipc/sourcerangescontainer.h
@@ -54,6 +54,11 @@ public:
         return m_sourceRangeWithTextContainers;
     }
 
+    std::vector<SourceRangeWithTextContainer> &sourceRangeWithTextContainers()
+    {
+        return m_sourceRangeWithTextContainers;
+    }
+
     std::vector<SourceRangeWithTextContainer> takeSourceRangeWithTextContainers()
     {
         return std::move(m_sourceRangeWithTextContainers);
diff --git a/src/libs/clangbackendipc/sourcerangewithtextcontainer.h b/src/libs/clangbackendipc/sourcerangewithtextcontainer.h
index 82d17f884a4..8a92086319b 100644
--- a/src/libs/clangbackendipc/sourcerangewithtextcontainer.h
+++ b/src/libs/clangbackendipc/sourcerangewithtextcontainer.h
@@ -115,3 +115,20 @@ using SourceRangeWithTextContainers = std::vector<SourceRangeWithTextContainer>;
 CMBIPC_EXPORT QDebug operator<<(QDebug debug, const SourceRangeWithTextContainer &container);
 std::ostream &operator<<(std::ostream &os, const SourceRangeWithTextContainer &container);
 } // namespace ClangBackEnd
+
+namespace std
+{
+template<> struct hash<ClangBackEnd::SourceRangeWithTextContainer>
+{
+    using argument_type = ClangBackEnd::SourceRangeWithTextContainer;
+    using result_type = std::size_t;
+    result_type operator()(const argument_type &container) const
+    {
+        const result_type h1{std::hash<uint>{}(container.fileHash())};
+        const result_type h2{std::hash<uint>{}(container.start().offset())};
+        const result_type h3{std::hash<uint>{}(container.end().offset())};
+
+        return h1 ^ (h2 << 8) ^ (h3 << 16);
+    }
+};
+}
diff --git a/src/tools/clangrefactoringbackend/source/clangquerygatherer.cpp b/src/tools/clangrefactoringbackend/source/clangquerygatherer.cpp
index 09c96a69b53..219011eff2a 100644
--- a/src/tools/clangrefactoringbackend/source/clangquerygatherer.cpp
+++ b/src/tools/clangrefactoringbackend/source/clangquerygatherer.cpp
@@ -34,6 +34,7 @@ ClangQueryGatherer::ClangQueryGatherer(StringCache<Utils::PathString, std::mutex
                                        std::vector<V2::FileContainer> &&unsaved,
                                        Utils::SmallString &&query)
     : m_filePathCache(filePathCache),
+      m_sourceRangeFilter(sources.size()),
       m_sources(std::move(sources)),
       m_unsaved(std::move(unsaved)),
       m_query(std::move(query))
@@ -125,7 +126,7 @@ std::vector<SourceRangesAndDiagnosticsForQueryMessage> ClangQueryGatherer::allCu
     std::vector<SourceRangesAndDiagnosticsForQueryMessage> messages;
 
     for (Future &future : m_sourceFutures)
-        messages.push_back(future.get());
+        messages.push_back(m_sourceRangeFilter.removeDuplicates(future.get()));
 
     return messages;
 }
@@ -135,7 +136,7 @@ std::vector<SourceRangesAndDiagnosticsForQueryMessage> ClangQueryGatherer::finis
     std::vector<SourceRangesAndDiagnosticsForQueryMessage> messages;
 
     for (auto &&future : finishedFutures())
-        messages.push_back(future.get());
+        messages.push_back(m_sourceRangeFilter.removeDuplicates(future.get()));
 
     return messages;
 }
diff --git a/src/tools/clangrefactoringbackend/source/clangquerygatherer.h b/src/tools/clangrefactoringbackend/source/clangquerygatherer.h
index b9370563791..67e662bd86c 100644
--- a/src/tools/clangrefactoringbackend/source/clangquerygatherer.h
+++ b/src/tools/clangrefactoringbackend/source/clangquerygatherer.h
@@ -25,6 +25,8 @@
 
 #pragma once
 
+#include "sourcerangefilter.h"
+
 #include <sourcerangesanddiagnosticsforquerymessage.h>
 #include <filecontainerv2.h>
 #include <stringcache.h>
@@ -68,6 +70,7 @@ protected:
 
 private:
     StringCache<Utils::PathString, std::mutex> *m_filePathCache = nullptr;
+    SourceRangeFilter m_sourceRangeFilter;
     std::vector<V2::FileContainer> m_sources;
     std::vector<V2::FileContainer> m_unsaved;
     Utils::SmallString m_query;
diff --git a/src/tools/clangrefactoringbackend/source/clangrefactoringbackend-source.pri b/src/tools/clangrefactoringbackend/source/clangrefactoringbackend-source.pri
index 7a695e5ca4d..8af6b89974c 100644
--- a/src/tools/clangrefactoringbackend/source/clangrefactoringbackend-source.pri
+++ b/src/tools/clangrefactoringbackend/source/clangrefactoringbackend-source.pri
@@ -1,7 +1,8 @@
 INCLUDEPATH += $$PWD
 
 HEADERS += \
-    $$PWD/clangrefactoringbackend_global.h
+    $$PWD/clangrefactoringbackend_global.h \
+    $$PWD/sourcerangefilter.h
 
 !isEmpty(LIBTOOLING_LIBS) {
 SOURCES += \
@@ -33,3 +34,6 @@ HEADERS += \
     $$PWD/locationsourcefilecallbacks.h \
     $$PWD/clangquerygatherer.h
 }
+
+SOURCES += \
+    $$PWD/sourcerangefilter.cpp
diff --git a/src/tools/clangrefactoringbackend/source/refactoringserver.cpp b/src/tools/clangrefactoringbackend/source/refactoringserver.cpp
index cf233baf34f..80fdbe86e3e 100644
--- a/src/tools/clangrefactoringbackend/source/refactoringserver.cpp
+++ b/src/tools/clangrefactoringbackend/source/refactoringserver.cpp
@@ -115,6 +115,11 @@ bool RefactoringServer::pollTimerIsActive() const
     return m_pollTimer.isActive();
 }
 
+void RefactoringServer::setGathererProcessingSlotCount(uint count)
+{
+    m_gatherer.setProcessingSlotCount(count);
+}
+
 void RefactoringServer::gatherSourceRangesAndDiagnosticsForQueryMessages(
         std::vector<V2::FileContainer> &&sources,
         std::vector<V2::FileContainer> &&unsaved,
diff --git a/src/tools/clangrefactoringbackend/source/refactoringserver.h b/src/tools/clangrefactoringbackend/source/refactoringserver.h
index 1befa076216..8926ce7c017 100644
--- a/src/tools/clangrefactoringbackend/source/refactoringserver.h
+++ b/src/tools/clangrefactoringbackend/source/refactoringserver.h
@@ -64,6 +64,8 @@ public:
 
     bool pollTimerIsActive() const;
 
+    void setGathererProcessingSlotCount(uint count);
+
 private:
     void gatherSourceRangesAndDiagnosticsForQueryMessages(std::vector<V2::FileContainer> &&sources,
                                                           std::vector<V2::FileContainer> &&unsaved,
diff --git a/src/tools/clangrefactoringbackend/source/sourcerangefilter.cpp b/src/tools/clangrefactoringbackend/source/sourcerangefilter.cpp
new file mode 100644
index 00000000000..bc566ea7d40
--- /dev/null
+++ b/src/tools/clangrefactoringbackend/source/sourcerangefilter.cpp
@@ -0,0 +1,59 @@
+/****************************************************************************
+**
+** Copyright (C) 2017 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#include "sourcerangefilter.h"
+
+#include <algorithm>
+
+namespace ClangBackEnd {
+
+SourceRangeFilter::SourceRangeFilter(std::size_t sourcesCount)
+{
+    m_collectedSourceRanges.reserve(sourcesCount);
+}
+
+SourceRangesAndDiagnosticsForQueryMessage SourceRangeFilter::removeDuplicates(SourceRangesAndDiagnosticsForQueryMessage &&message)
+{
+    removeDuplicates(message.sourceRanges().sourceRangeWithTextContainers());
+
+    return std::move(message);
+}
+
+void SourceRangeFilter::removeDuplicates(SourceRangeWithTextContainers &sourceRanges)
+{
+    auto partitionPoint = std::stable_partition(sourceRanges.begin(),
+                                                sourceRanges.end(),
+                                                [&] (const SourceRangeWithTextContainer &sourceRange) {
+        return m_collectedSourceRanges.find(sourceRange) == m_collectedSourceRanges.end();
+    });
+
+    sourceRanges.erase(partitionPoint, sourceRanges.end());
+
+    std::copy(sourceRanges.begin(),
+              sourceRanges.end(),
+              std::inserter(m_collectedSourceRanges, m_collectedSourceRanges.end()));
+}
+
+} // namespace ClangBackEnd
diff --git a/src/tools/clangrefactoringbackend/source/sourcerangefilter.h b/src/tools/clangrefactoringbackend/source/sourcerangefilter.h
new file mode 100644
index 00000000000..8c6c3ac42f8
--- /dev/null
+++ b/src/tools/clangrefactoringbackend/source/sourcerangefilter.h
@@ -0,0 +1,47 @@
+/****************************************************************************
+**
+** Copyright (C) 2017 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#pragma once
+
+#include <unordered_set>
+
+#include <sourcerangesanddiagnosticsforquerymessage.h>
+
+namespace ClangBackEnd {
+
+class SourceRangeFilter
+{
+public:
+    SourceRangeFilter(std::size_t sourcesCount = 0);
+
+    SourceRangesAndDiagnosticsForQueryMessage
+    removeDuplicates(SourceRangesAndDiagnosticsForQueryMessage &&message);
+    void removeDuplicates(SourceRangeWithTextContainers &sourceRanges);
+
+private:
+    std::unordered_set<SourceRangeWithTextContainer> m_collectedSourceRanges;
+};
+
+} // namespace ClangBackEnd
diff --git a/tests/unit/unittest/clangquerygatherer-test.cpp b/tests/unit/unittest/clangquerygatherer-test.cpp
index 0e24e4fa6d4..b4808b45c57 100644
--- a/tests/unit/unittest/clangquerygatherer-test.cpp
+++ b/tests/unit/unittest/clangquerygatherer-test.cpp
@@ -38,6 +38,7 @@ using testing::AtLeast;
 using testing::AtMost;
 using testing::Contains;
 using testing::Each;
+using testing::ElementsAre;
 using testing::Eq;
 using testing::Ge;
 using testing::IsEmpty;
@@ -47,6 +48,7 @@ using testing::Pair;
 using testing::PrintToString;
 using testing::Property;
 using testing::SizeIs;
+using testing::UnorderedElementsAre;
 using testing::_;
 
 using ClangBackEnd::V2::FileContainer;
@@ -72,10 +74,16 @@ protected:
 
 protected:
     ClangBackEnd::StringCache<Utils::PathString, std::mutex> filePathCache;
-    Utils::SmallString sourceContent{"#include \"query_simplefunction.h\"\nvoid f()\n {}"};
+    Utils::SmallString sourceContent{"#include \"query_simplefunction.h\"\nvoid f() {}"};
     FileContainer source{{TESTDATA_DIR, "query_simplefunction.cpp"},
                          sourceContent.clone(),
-                         {"cc", "query_simplefunction.cpp"}};
+                         {"cc", TESTDATA_DIR"/query_simplefunction.cpp", "-I", TESTDATA_DIR}};
+    FileContainer source2{{TESTDATA_DIR, "query_simplefunction2.cpp"},
+                          {},
+                          {"cc", TESTDATA_DIR"/query_simplefunction2.cpp", "-I", TESTDATA_DIR}};
+    FileContainer source3{{TESTDATA_DIR, "query_simplefunction3.cpp"},
+                          {},
+                          {"cc", TESTDATA_DIR"/query_simplefunction3.cpp", "-I", TESTDATA_DIR}};
     Utils::SmallString unsavedContent{"void f();"};
     FileContainer unsaved{{TESTDATA_DIR, "query_simplefunction.h"},
                           unsavedContent.clone(),
@@ -83,19 +91,19 @@ protected:
     Utils::SmallString query{"functionDecl()"};
     ClangBackEnd::ClangQueryGatherer gatherer{&filePathCache, {source.clone()}, {unsaved.clone()}, query.clone()};
     ClangBackEnd::ClangQueryGatherer manyGatherer{&filePathCache,
-                                                  {source.clone(), source.clone(), source.clone()},
+                                                  {source3.clone(), source2.clone(), source.clone()},
                                                   {unsaved.clone()},
                                                   query.clone()};
 };
 
 TEST_F(ClangQueryGatherer, CreateSourceRangesAndDiagnostics)
 {
-    auto sourceRangesAndDiagnostics = gatherer.createSourceRangesAndDiagnosticsForSource(&filePathCache, source.clone(), {}, query.clone());
+    auto sourceRangesAndDiagnostics = gatherer.createSourceRangesAndDiagnosticsForSource(&filePathCache, source.clone(), {unsaved}, query.clone());
 
     ASSERT_THAT(sourceRangesAndDiagnostics,
                 Property(&SourceRangesAndDiagnosticsForQueryMessage::sourceRanges,
                          Property(&SourceRangesContainer::sourceRangeWithTextContainers,
-                                  Contains(IsSourceRangeWithText(2, 1, 3, 4, "void f()\n {}")))));
+                                  Contains(IsSourceRangeWithText(2, 1, 2, 12, "void f() {}")))));
 }
 
 TEST_F(ClangQueryGatherer, CreateSourceRangesAndDiagnosticssWithUnsavedContent)
@@ -176,14 +184,18 @@ TEST_F(ClangQueryGatherer, AfterStartCreateSourceRangesAndDiagnosticsMessagesGet
     manyGatherer.startCreateNextSourceRangesAndDiagnosticsMessages();
 
     ASSERT_THAT(manyGatherer.allCurrentProcessedMessages(),
-                Each(
+                UnorderedElementsAre(
                     Property(&SourceRangesAndDiagnosticsForQueryMessage::sourceRanges,
                              Property(&SourceRangesContainer::sourceRangeWithTextContainers,
-                                      Contains(IsSourceRangeWithText(1, 1, 1, 9, "void f();"))))));
+                                      UnorderedElementsAre(IsSourceRangeWithText(1, 1, 1, 9, "void f();"),
+                                                           IsSourceRangeWithText(2, 1, 2, 12, "void f() {}")))),
+                    Property(&SourceRangesAndDiagnosticsForQueryMessage::sourceRanges,
+                             Property(&SourceRangesContainer::sourceRangeWithTextContainers,
+                                      UnorderedElementsAre(
+                                          IsSourceRangeWithText(1, 1, 1, 13, "int header();"),
+                                          IsSourceRangeWithText(3, 1, 3, 15, "int function();"))))));
 }
 
-
-
 TEST_F(ClangQueryGatherer, GetFinishedMessages)
 {
     manyGatherer.startCreateNextSourceRangesAndDiagnosticsMessages();
@@ -193,10 +205,17 @@ TEST_F(ClangQueryGatherer, GetFinishedMessages)
 
     ASSERT_THAT(messages,
                 AllOf(SizeIs(2),
-                      Each(
+                      UnorderedElementsAre(
                           Property(&SourceRangesAndDiagnosticsForQueryMessage::sourceRanges,
                                    Property(&SourceRangesContainer::sourceRangeWithTextContainers,
-                                            Contains(IsSourceRangeWithText(1, 1, 1, 9, "void f();")))))));
+                                            UnorderedElementsAre(
+                                                IsSourceRangeWithText(1, 1, 1, 9, "void f();"),
+                                                IsSourceRangeWithText(2, 1, 2, 12, "void f() {}")))),
+                          Property(&SourceRangesAndDiagnosticsForQueryMessage::sourceRanges,
+                                   Property(&SourceRangesContainer::sourceRangeWithTextContainers,
+                                            UnorderedElementsAre(
+                                                IsSourceRangeWithText(1, 1, 1, 13, "int header();"),
+                                                IsSourceRangeWithText(3, 1, 3, 15, "int function();")))))));
 }
 
 TEST_F(ClangQueryGatherer, GetFinishedMessagesAfterSecondPass)
@@ -211,10 +230,38 @@ TEST_F(ClangQueryGatherer, GetFinishedMessagesAfterSecondPass)
 
     ASSERT_THAT(messages,
                 AllOf(SizeIs(1),
-                      Each(
+                      ElementsAre(
                           Property(&SourceRangesAndDiagnosticsForQueryMessage::sourceRanges,
                                    Property(&SourceRangesContainer::sourceRangeWithTextContainers,
-                                            Contains(IsSourceRangeWithText(1, 1, 1, 9, "void f();")))))));
+                                            UnorderedElementsAre(
+                                                IsSourceRangeWithText(3, 1, 3, 15, "int function();")))))));
+}
+
+TEST_F(ClangQueryGatherer, FilterDuplicates)
+{
+    manyGatherer.setProcessingSlotCount(3);
+    manyGatherer.startCreateNextSourceRangesAndDiagnosticsMessages();
+    manyGatherer.waitForFinished();
+
+    auto messages = manyGatherer.finishedMessages();
+
+    ASSERT_THAT(messages,
+                AllOf(SizeIs(3),
+                      UnorderedElementsAre(
+                          Property(&SourceRangesAndDiagnosticsForQueryMessage::sourceRanges,
+                                   Property(&SourceRangesContainer::sourceRangeWithTextContainers,
+                                            UnorderedElementsAre(
+                                                IsSourceRangeWithText(1, 1, 1, 9, "void f();"),
+                                                IsSourceRangeWithText(2, 1, 2, 12, "void f() {}")))),
+                          Property(&SourceRangesAndDiagnosticsForQueryMessage::sourceRanges,
+                                   Property(&SourceRangesContainer::sourceRangeWithTextContainers,
+                                            UnorderedElementsAre(
+                                                IsSourceRangeWithText(1, 1, 1, 13, "int header();"),
+                                                IsSourceRangeWithText(3, 1, 3, 15, "int function();")))),
+                          Property(&SourceRangesAndDiagnosticsForQueryMessage::sourceRanges,
+                                   Property(&SourceRangesContainer::sourceRangeWithTextContainers,
+                                            UnorderedElementsAre(
+                                                IsSourceRangeWithText(3, 1, 3, 15, "int function();")))))));
 }
 
 TEST_F(ClangQueryGatherer, AfterGetFinishedMessagesFuturesAreReduced)
diff --git a/tests/unit/unittest/data/query_simplefunction2.cpp b/tests/unit/unittest/data/query_simplefunction2.cpp
new file mode 100644
index 00000000000..fbf4d8d8fd0
--- /dev/null
+++ b/tests/unit/unittest/data/query_simplefunction2.cpp
@@ -0,0 +1,4 @@
+#include "query_simplefunction2.h"
+
+int function();
+
diff --git a/tests/unit/unittest/data/query_simplefunction2.h b/tests/unit/unittest/data/query_simplefunction2.h
new file mode 100644
index 00000000000..9251cf100a9
--- /dev/null
+++ b/tests/unit/unittest/data/query_simplefunction2.h
@@ -0,0 +1,2 @@
+int header();
+
diff --git a/tests/unit/unittest/data/query_simplefunction3.cpp b/tests/unit/unittest/data/query_simplefunction3.cpp
new file mode 100644
index 00000000000..fbf4d8d8fd0
--- /dev/null
+++ b/tests/unit/unittest/data/query_simplefunction3.cpp
@@ -0,0 +1,4 @@
+#include "query_simplefunction2.h"
+
+int function();
+
diff --git a/tests/unit/unittest/refactoringserver-test.cpp b/tests/unit/unittest/refactoringserver-test.cpp
index 33b63534e3f..39db78bdcd0 100644
--- a/tests/unit/unittest/refactoringserver-test.cpp
+++ b/tests/unit/unittest/refactoringserver-test.cpp
@@ -40,6 +40,7 @@ namespace {
 using testing::AllOf;
 using testing::Contains;
 using testing::NiceMock;
+using testing::Not;
 using testing::Pair;
 using testing::PrintToString;
 using testing::Property;
@@ -77,7 +78,8 @@ protected:
     Utils::SmallString sourceContent{"void f()\n {}"};
     FileContainer source{{TESTDATA_DIR, "query_simplefunction.cpp"},
                          sourceContent.clone(),
-                         {"cc", "query_simplefunction.cpp"}};
+                         {"cc", TESTDATA_DIR"/query_simplefunction.cpp"}};
+    int processingSlotCount = 2;
 };
 
 using RefactoringServerSlowTest = RefactoringServer;
@@ -153,8 +155,12 @@ TEST_F(RefactoringServerSlowTest, RequestTwoSourceRangesAndDiagnosticsForQueryMe
                 sourceRangesAndDiagnosticsForQueryMessage(
                     Property(&SourceRangesAndDiagnosticsForQueryMessage::sourceRanges,
                              Property(&SourceRangesContainer::sourceRangeWithTextContainers,
-                                      Contains(IsSourceRangeWithText(1, 1, 2, 4, sourceContent))))))
-            .Times(2);
+                                      Contains(IsSourceRangeWithText(1, 1, 2, 4, sourceContent))))));
+    EXPECT_CALL(mockRefactoringClient,
+                sourceRangesAndDiagnosticsForQueryMessage(
+                    Property(&SourceRangesAndDiagnosticsForQueryMessage::sourceRanges,
+                             Property(&SourceRangesContainer::sourceRangeWithTextContainers,
+                                      Not(Contains(IsSourceRangeWithText(1, 1, 2, 4, sourceContent)))))));
 
     refactoringServer.requestSourceRangesAndDiagnosticsForQueryMessage(std::move(requestSourceRangesAndDiagnosticsForQueryMessage));
 }
@@ -163,7 +169,7 @@ TEST_F(RefactoringServerVerySlowTest, RequestManySourceRangesAndDiagnosticsForQu
 {
     std::vector<FileContainer> sources;
     std::fill_n(std::back_inserter(sources),
-                std::thread::hardware_concurrency() + 3,
+                processingSlotCount + 3,
                 source.clone());
     RequestSourceRangesAndDiagnosticsForQueryMessage requestSourceRangesAndDiagnosticsForQueryMessage{"functionDecl()",
                                                                                                       std::move(sources),
@@ -173,8 +179,13 @@ TEST_F(RefactoringServerVerySlowTest, RequestManySourceRangesAndDiagnosticsForQu
                 sourceRangesAndDiagnosticsForQueryMessage(
                     Property(&SourceRangesAndDiagnosticsForQueryMessage::sourceRanges,
                              Property(&SourceRangesContainer::sourceRangeWithTextContainers,
-                                      Contains(IsSourceRangeWithText(1, 1, 2, 4, sourceContent))))))
-            .Times(std::thread::hardware_concurrency() + 3);
+                                      Contains(IsSourceRangeWithText(1, 1, 2, 4, sourceContent))))));
+    EXPECT_CALL(mockRefactoringClient,
+                sourceRangesAndDiagnosticsForQueryMessage(
+                    Property(&SourceRangesAndDiagnosticsForQueryMessage::sourceRanges,
+                             Property(&SourceRangesContainer::sourceRangeWithTextContainers,
+                                      Not(Contains(IsSourceRangeWithText(1, 1, 2, 4, sourceContent)))))))
+            .Times(processingSlotCount + 2);
 
     refactoringServer.requestSourceRangesAndDiagnosticsForQueryMessage(std::move(requestSourceRangesAndDiagnosticsForQueryMessage));
 }
@@ -237,6 +248,7 @@ void RefactoringServer::SetUp()
 
 void RefactoringServer::TearDown()
 {
+    refactoringServer.setGathererProcessingSlotCount(uint(processingSlotCount));
     refactoringServer.waitThatSourceRangesAndDiagnosticsForQueryMessagesAreFinished();
 }
 
diff --git a/tests/unit/unittest/sourcerangefilter-test.cpp b/tests/unit/unittest/sourcerangefilter-test.cpp
new file mode 100644
index 00000000000..059f9827f62
--- /dev/null
+++ b/tests/unit/unittest/sourcerangefilter-test.cpp
@@ -0,0 +1,106 @@
+/****************************************************************************
+**
+** Copyright (C) 2017 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#include "googletest.h"
+
+#include <sourcerangefilter.h>
+
+namespace {
+
+using testing::ContainerEq;
+using testing::IsEmpty;
+
+using ClangBackEnd::SourceRangeWithTextContainer;
+using ClangBackEnd::SourceRangeWithTextContainers;
+using ClangBackEnd::SourceRangesAndDiagnosticsForQueryMessage;
+
+class SourceRangeFilter : public ::testing::Test
+{
+protected:
+
+
+protected:
+    SourceRangeWithTextContainers sourceRanges1{{1, 1, 1, 1, 2, 1, 4, "foo"},
+                                               {2, 1, 1, 1, 2, 1, 4, "foo"},
+                                               {1, 1, 1, 1, 2, 2, 5, "foo"}};
+    SourceRangeWithTextContainers sourceRanges2{{1, 1, 1, 1, 2, 1, 4, "foo"},
+                                                {3, 1, 1, 1, 2, 1, 4, "foo"},
+                                                {1, 1, 1, 1, 2, 2, 6, "foo"}};
+    SourceRangeWithTextContainers sourceRanges3{{3, 1, 1, 1, 2, 1, 4, "foo"},
+                                                {1, 1, 1, 1, 2, 2, 6, "foo"}};
+    SourceRangesAndDiagnosticsForQueryMessage message1{{{}, Utils::clone(sourceRanges1)}, {}};
+    SourceRangesAndDiagnosticsForQueryMessage message2{{{}, Utils::clone(sourceRanges2)}, {}};
+    ClangBackEnd::SourceRangeFilter filter{3};
+};
+
+TEST_F(SourceRangeFilter, DontChangeForFirstTime)
+{
+    auto expectedSourceRanges = sourceRanges1;
+
+    filter.removeDuplicates(sourceRanges1);
+
+    ASSERT_THAT(sourceRanges1, ContainerEq(expectedSourceRanges));
+}
+
+TEST_F(SourceRangeFilter, DoNotFilterNonDuplicates)
+{
+    SourceRangeWithTextContainers expectedSourceRanges = sourceRanges3;
+    filter.removeDuplicates(sourceRanges1);
+
+    filter.removeDuplicates(sourceRanges3);
+
+    ASSERT_THAT(sourceRanges3, ContainerEq(expectedSourceRanges));
+}
+
+TEST_F(SourceRangeFilter, FilterDuplicates)
+{
+    filter.removeDuplicates(sourceRanges1);
+
+    filter.removeDuplicates(sourceRanges2);
+
+    ASSERT_THAT(sourceRanges2, ContainerEq(sourceRanges3));
+}
+
+TEST_F(SourceRangeFilter, FilterMoreDuplicates)
+{
+    filter.removeDuplicates(sourceRanges1);
+    filter.removeDuplicates(sourceRanges2);
+
+    filter.removeDuplicates(sourceRanges3);
+
+    ASSERT_THAT(sourceRanges3, IsEmpty());
+}
+
+TEST_F(SourceRangeFilter, FilterDuplicatesFromMessage)
+{
+    filter.removeDuplicates(std::move(message1));
+
+    auto filteredMessage = filter.removeDuplicates(std::move(message2));
+
+    ASSERT_THAT(filteredMessage.sourceRanges().sourceRangeWithTextContainers(),
+                ContainerEq(sourceRanges3));
+}
+
+}
diff --git a/tests/unit/unittest/unittest.pro b/tests/unit/unittest/unittest.pro
index 4e32ce0413c..8409517e8f3 100644
--- a/tests/unit/unittest/unittest.pro
+++ b/tests/unit/unittest/unittest.pro
@@ -64,7 +64,8 @@ SOURCES += \
     pchmanagerserver-test.cpp \
     pchmanagerclientserverinprocess-test.cpp \
     clangquerygatherer-test.cpp \
-    filepath-test.cpp
+    filepath-test.cpp \
+    sourcerangefilter-test.cpp
 
 !isEmpty(LIBCLANG_LIBS) {
 SOURCES += \