有个二维数组，如let arr = [[{id:1, age:10}, {id:2, age:12}],[{id:3, age:14}, {id:4,age:15}]]，现用

<block wx:for="{{arr}}" wx:key="groupIndex" wx:for-index="groupIndex" wx:for-item="groupItem">

<block wx:for="{{groupItem}}" wx:key="id" wx:for-index="subIndex" wx:for-item="subItem">

<block>

<block>

在分页加载数据时，会有如下提示：[/__pageframe__/pages/tabFourth/tabFourth] Do not set same key \"[object Object]\" in wx:key.

想问第一层循环中的wx:key该如何设置(无论wx:key="*this"，还是wx:key="groupIndex"，都会报上面警告)