diff --git a/src/os_unix.h b/src/os_unix.h
index d025200..4a35338 100644
--- a/src/os_unix.h
+++ b/src/os_unix.h
@@ -565,7 +565,9 @@
 # endif
 #endif
 
-#define HAVE_DUP		/* have dup() */
+#ifndef HAVE_DUP
+# define HAVE_DUP		/* have dup() */
+#endif
 #define HAVE_ST_MODE		/* have stat.st_mode */
 
 /* We have three kinds of ACL support. */