avoid calling duplicate enter/leave when already in transition (fix #3252)

Author: yyx990803

src/platforms/web/runtime/modules/transition.js

@@ -20,6 +20,10 @@ export function enter (vnode: VNodeWithData) {
     el._leaveCb()
   }
 
+  if (el._enterCb) {
+    return
+  }
+
   const data = resolveTransition(vnode.data.transition)
   if (!data) {
     return
@@ -112,6 +116,10 @@ export function leave (vnode: VNodeWithData, rm: Function) {
     el._enterCb()
   }
 
+  if (el._leaveCb) {
+    return
+  }
+
   const data = resolveTransition(vnode.data.transition)
   if (!data) {
     return rm()