Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -201,6 +201,10 @@
 include(config-ix)
 
 if(MSVC)
+  # MSVC will give a warning if there is already another warning option.
+  # Remove any such option if present before appending /W3.
+  string(REGEX REPLACE " /W[0-4]" "" CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
+  string(REGEX REPLACE " /W[0-4]" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
   append_string_if(COMPILER_RT_HAS_W3_FLAG /W3 CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 else()
   append_string_if(COMPILER_RT_HAS_WALL_FLAG -Wall CMAKE_C_FLAGS CMAKE_CXX_FLAGS)