ng-repeat循环输出出现Duplicates in a repeater are not allowed. Use 'track by' expression to specify unique

   使用ng-repeat循环输出对象的时候出现如下错误：

Duplicates in a repeater are not allowed. Use ‘track by‘ expression to specify unique keys. Repeater: c in shopCount, Duplicate key: undefined:undefined

应该在循环是加以下代码    ng-repeat item in items track by $index