Index: src/wp-admin/includes/post.php
===================================================================
--- src/wp-admin/includes/post.php	(revision 32914)
+++ src/wp-admin/includes/post.php	(working copy)
@@ -819,7 +819,7 @@
 
 		$metakey = wp_slash( $metakey );
 
-		return add_post_meta( $post_ID, $metakey, $metavalue );
+		return update_post_meta( $post_ID, $metakey, $metavalue );
 	}
 
 	return false;