diff --git a/include/r3_str.h b/include/r3_str.h
index 6771dde..974867a 100644
--- a/include/r3_str.h
+++ b/include/r3_str.h
@@ -24,16 +24,20 @@ void print_indent(int level);
 
 
 #if _GNU_SOURCE || POSIX_C_SOURCE >= 200809L || _XOPEN_SOURCE >= 700 || __DARWIN_C_LEVEL >= 200809L
+#ifndef HAVE_STRNDUP
 #define HAVE_STRNDUP
 #endif
+#endif
 
 #if _SVID_SOURCE || _BSD_SOURCE \
     || _XOPEN_SOURCE >= 500 \
     || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED \
     || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L \
     || __DARWIN_C_LEVEL >= 200809L
+#ifndef HAVE_STRDUP
 #define HAVE_STRDUP
 #endif
+#endif
 
 #ifndef HAVE_STRDUP
 char *strdup(const char *s);