diff --git a/configure/os/CONFIG.Common.win32-x86-mingw b/configure/os/CONFIG.Common.win32-x86-mingw
index e3ec3fc5c..262129024 100644
--- a/configure/os/CONFIG.Common.win32-x86-mingw
+++ b/configure/os/CONFIG.Common.win32-x86-mingw
@@ -34,7 +34,7 @@ ARCH_DEP_LDFLAGS += -m32
 EXE=.exe
 
 # Override for -DUNIX from CONFIG.Common.UnixCommon
-OP_SYS_CPPFLAGS += -D_MINGW
+OP_SYS_CPPFLAGS = -D_MINGW
 
 VISC_DLL_NO = -DEPICS_DLL_NO
 VISC_DLL_YES =