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>
 
diff --git a/library/x509_crt.c b/library/x509_crt.c
index 2fd56fb..53cdcf0 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -48,7 +48,9 @@
 
 #if defined(MBEDTLS_HAVE_TIME)
 #if defined(_WIN32) && !defined(EFIX64) && !defined(EFI32)
+#ifndef WIN32_LEAN_AND_MEAN
 #define WIN32_LEAN_AND_MEAN
+#endif
 #include <windows.h>
 #else
 #include <time.h>