diff --git a/platformio/__init__.py b/platformio/__init__.py
index b2567541..818bf941 100644
--- a/platformio/__init__.py
+++ b/platformio/__init__.py
@@ -14,7 +14,7 @@
 
 import sys
 
-VERSION = (2, 10, "4.dev1")
+VERSION = (2, 10, "4.dev2")
 __version__ = ".".join([str(s) for s in VERSION])
 
 __title__ = "platformio"
diff --git a/platformio/builder/tools/platformio.py b/platformio/builder/tools/platformio.py
index 41341b96..22e7dc94 100644
--- a/platformio/builder/tools/platformio.py
+++ b/platformio/builder/tools/platformio.py
@@ -159,9 +159,9 @@ def ProcessUnFlags(env, flags):
 
     for key in parsed_flags.keys():
         cur_flags = set(env.get(key, []))
-        common = cur_flags & all_flags
-        for item in common:
-            env[key].remove(item)
+        for item in (cur_flags & all_flags):
+            while item in env[key]:
+                env[key].remove(item)
 
 
 def IsFileWithExt(env, file_, ext):  # pylint: disable=W0613