diff --git a/src/include/port/win32_port.h b/src/include/port/win32_port.h
index 5ea66528fa..4de5bf3bf6 100644
--- a/src/include/port/win32_port.h
+++ b/src/include/port/win32_port.h
@@ -87,7 +87,9 @@
  * We have a replacement for fdatasync() in src/port/fdatasync.c, which is
  * unconditionally used by MSVC and Mingw builds.
  */
+#ifndef HAVE_FDATASYNC
 #define HAVE_FDATASYNC
+#endif
 
 #define USES_WINSOCK