You can track by $index if your data source has duplicate identifiers
e.g.: $scope.dataSource: [{id:1,name:'one'}, {id:1,name:'one too'}, {id:2,name:'two'}]
You can't iterate this collection while using 'id' as identifier (duplicate id:1).
WON'T WORK:
<element ng-repeat="item.id as item.name for item in dataSource">
// something with item ...
</element>
but you can, if using track by $index:
<element ng-repeat="item in dataSource track by $index">
// something with item ...
</element>