avoid pushing the same ref more than once (fix #4253)

src/core/vdom/modules/ref.js

@@ -32,7 +32,7 @@ export function registerRef (vnode: VNodeWithData, isRemoval: ?boolean) {
     }
   } else {
     if (vnode.data.refInFor) {
-      if (Array.isArray(refs[key])) {
+      if (Array.isArray(refs[key]) && refs[key].indexOf(ref) < 0) {
         refs[key].push(ref)
       } else {
         refs[key] = [ref]