angularjs - "track by $index" creating duplicates on save using ng-repeat -

i'm having problem when update object, it's creating duplicate of instance (singleorder) in model, instead of updating exisiting one. can't clear whole $scope , reload, i'm updating part of object.

here's save function:

$scope.savedetails = function(singleorder, data, status) {                    ordersservice.updateorders(singleorder.order)      .success(function(data, status) {         angular.extend(singleorder, data);                           })                              } 

where singleorder relates instance of main orderdetails object, used in ng-repeat

<div ng-repeat="singleorder in orderdetails track $index"> 

here's function called in ordersservice, if helps:

getdata.updateorders = function(data){     var url = '/service/rest/orders';      return $http.post(url, data)                     }; 

thanks can offer.

update: i've tried messing around track by , received following results:

<div ng-repeat="singleorder in orderdetails track $index"> saves content fine, creates duplicate order on dom.

<div ng-repeat="singleorder in orderdetails track singleorder.order.id"> works each order has unique id. can save fine , doesn't create duplicate in dom, duplicates in repeater not allowed in console.

confused why happening. ideas welcome!

angular.extend() returns destination object, maybe this

ordersservice.updateorders(singleorder.order) .success(function(data, status) {     singleorder = angular.extend(singleorder, data);                       }) 

but in opinion better when server returns whole object client needs. code easier.