diff --git a/CHANGELOG.md b/CHANGELOG.md
index 867227fd..62dbdf4b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@ HEAD
 ----
 
 * Changed the rules of string duplication (issue #658)
+* Changed the return type of `strdup()` to `const char*` to prevent double duplication
 
 > ### New rules for string duplication
 >
diff --git a/src/ArduinoJson/JsonBuffer.hpp b/src/ArduinoJson/JsonBuffer.hpp
index 5a42a7d0..36775cb1 100644
--- a/src/ArduinoJson/JsonBuffer.hpp
+++ b/src/ArduinoJson/JsonBuffer.hpp
@@ -38,19 +38,19 @@ class JsonBuffer : Internals::NonCopyable {
 
   // Duplicates a string
   //
-  // char* strdup(TValue);
+  // const char* strdup(TValue);
   // TValue = const std::string&, const String&,
   template <typename TString>
   typename TypeTraits::EnableIf<!TypeTraits::IsArray<TString>::value,
-                                char *>::type
+                                const char *>::type
   strdup(const TString &src) {
     return Internals::StringTraits<TString>::duplicate(src, this);
   }
   //
-  // char* strdup(TValue);
+  // const char* strdup(TValue);
   // TValue = const char*, const char[N], const FlashStringHelper*
   template <typename TString>
-  char *strdup(const TString *src) {
+  const char *strdup(const TString *src) {
     return Internals::StringTraits<const TString *>::duplicate(src, this);
   }
 
diff --git a/test/DynamicJsonBuffer/strdup.cpp b/test/DynamicJsonBuffer/strdup.cpp
index 9feea421..f6fb3b1c 100644
--- a/test/DynamicJsonBuffer/strdup.cpp
+++ b/test/DynamicJsonBuffer/strdup.cpp
@@ -10,14 +10,14 @@ TEST_CASE("DynamicJsonBuffer::strdup()") {
 
   SECTION("Should return a copy") {
     char original[] = "hello";
-    char* copy = buffer.strdup(original);
+    const char* copy = buffer.strdup(original);
     strcpy(original, "world");
     REQUIRE(std::string("hello") == copy);
   }
 
   SECTION("Given NULL, return NULL") {
     const char* original = NULL;
-    char* copy = buffer.strdup(original);
+    const char* copy = buffer.strdup(original);
     REQUIRE(0 == copy);
   }
 }
diff --git a/test/Misc/std_string.cpp b/test/Misc/std_string.cpp
index 00b76e83..2cfa3a3c 100644
--- a/test/Misc/std_string.cpp
+++ b/test/Misc/std_string.cpp
@@ -243,7 +243,7 @@ TEST_CASE("std::string") {
 
   SECTION("JsonBuffer_strdup") {
     std::string original("hello");
-    char *copy = jb.strdup(original);
+    const char *copy = jb.strdup(original);
     original[0] = 'w';
     REQUIRE(std::string("hello") == copy);
   }