diff --git a/src/init.c b/src/init.c
index 97a50f4a..2acbc433 100644
--- a/src/init.c
+++ b/src/init.c
@@ -45,9 +45,11 @@ SEXP is_asan_()
 {
 #if defined(__has_feature)
 #if __has_feature(address_sanitizer) // for clang
+#if !defined(__SANITIZE_ADDRESS__)
 #define __SANITIZE_ADDRESS__         // GCC already sets this
 #endif
 #endif
+#endif
 
 #ifdef __SANITIZE_ADDRESS__
   return Rf_ScalarLogical(1);