diff --git a/compiler-rt/CMakeLists.txt b/compiler-rt/CMakeLists.txt
index cc3190bd7f761..514ac250075a8 100644
--- a/compiler-rt/CMakeLists.txt
+++ b/compiler-rt/CMakeLists.txt
@@ -375,6 +375,25 @@ endif()
 if(MSVC)
   # FIXME: In fact, sanitizers should support both /MT and /MD, see PR20214.
   set(CMAKE_MSVC_RUNTIME_LIBRARY MultiThreaded)
+
+  # Remove any /M[DT][d] flags, and strip any definitions of _DEBUG.
+  # Since we're using CMAKE_MSVC_RUNTIME_LIBRARY (CMP0091 set to NEW),
+  # these options shouldn't be included in these flags variables. However,
+  # package managers that don't know which mechanism is used for passing
+  # CRT choice flags might be passing them both ways - which leads to
+  # duplicate CRT choice options. Thus make sure to strip out these flags
+  # from these variables, when we're forcing a CRT choice other than what
+  # the user requested here.
+  foreach(flag_var
+    CMAKE_C_FLAGS CMAKE_C_FLAGS_DEBUG CMAKE_C_FLAGS_RELEASE
+    CMAKE_C_FLAGS_MINSIZEREL CMAKE_C_FLAGS_RELWITHDEBINFO
+    CMAKE_CXX_FLAGS CMAKE_CXX_FLAGS_DEBUG CMAKE_CXX_FLAGS_RELEASE
+    CMAKE_CXX_FLAGS_MINSIZEREL CMAKE_CXX_FLAGS_RELWITHDEBINFO)
+    string(REGEX REPLACE "[/-]M[DT]d" "" ${flag_var} "${${flag_var}}")
+    string(REGEX REPLACE "[/-]MD" "" ${flag_var} "${${flag_var}}")
+    string(REGEX REPLACE "[/-]D_DEBUG" "" ${flag_var} "${${flag_var}}")
+  endforeach()
+
   append_list_if(COMPILER_RT_HAS_Oy_FLAG /Oy- SANITIZER_COMMON_CFLAGS)
   append_list_if(COMPILER_RT_HAS_GS_FLAG /GS- SANITIZER_COMMON_CFLAGS)