Ion-item and track by $index

ionic-v1
1

How do I use track by $index on an ion item like this:

<ion-item ng-repeat="item in items track by $index">{{item }}</ion-item>

Scope

 $scope.items = [1,1,1,2,3,4,5,5,6];

I’m getting this error:

Error: [ngRepeat:dupes] Duplicates in a repeater are not allowed. Use 'track by' expression to specify unique keys. Repeater: item in items , Duplicate key: object:03M