diff --git a/library/sha256.c b/library/sha256.c
index 8b2c345..159accc 100644
--- a/library/sha256.c
+++ b/library/sha256.c
@@ -152,7 +152,9 @@
     return 1;
 }
 #elif defined(MBEDTLS_PLATFORM_IS_WINDOWS_ON_ARM64)
+#ifndef WIN32_LEAN_AND_MEAN
 #define WIN32_LEAN_AND_MEAN
+#endif
 #include <Windows.h>
 #include <processthreadsapi.h>