using push function in angular js

i am trying to make a eCommerce shop with angular and code is as follow

var shopApp = angular.module('shopApp', ["slugifier"], function() {});

   controllers.productController = function($scope,FetchFactory) {

     $scope.fetchProducts = function(offset) {
        FetchFactory.items.fetchItems(offset).then(function(data){
        $scope.products = data;
    });
  }
  var activeAttrValue = 0;
  var activeAttrUnit = 0;
  var activeAttrId = 0;
   $scope.add_to_cart() = function(index){
        var newProd = [];
        newProd = $scope.products[index]; // $scope.products have products json
        newProd.quantity = 1;
        newProd.attr_id = activeAttrId;
        newProd.attr_value = activeAttrValue;
        newProd.attr_unit = activeAttrUnit;
        $scope.cartProducts.itemlist.push(newProd);
        $scope.cartProducts.total_items++;
        $scope.cartProducts.total_price += parseInt(newProd.price);
   }
 }
 shopApp.controller(controllers);

i have a made a factory which will return me products json after a ajax request and using ng-repeat as "product in products" i am displaying all the products in my html and i have make a shopping cart where the products that are added to cart are being displayed by ng-repeat of "cartProduct in cartProducts.itemlist track by $index" products have attributes like color,size,weight etc. whenever a attribute is clicked its value(like weight) and unit(kg) are store in a global variables activeAttrValue,activeAttrUnit and if add to cart is clicked they are stored in cartProducts.

Problem: In case of multiple attributes. when one attribute is selected added to cart, and again another attribute of same product is selected and added to cart.ideally they should add to cart as two products but instead last added product and attributes only adds and in console gives a error

 Error: [ngRepeat:dupes] http://errors.angularjs.org/1.2.16/ngRepeat/dupes?p0=cartProduct%20in%20cartProducts.itemlist&p1=object%3A006

when i tried console.log($scope.cartProducts.itemlist); i got this

 Object
   $$hashKey: "005"
   attr_id: "3"
   attr_unit: ""
   attr_value: "blue"
  , 
  Object
   $$hashKey: "005"
   attr_id: "3"
   attr_unit: ""
   attr_value: "blue"

attributes of both products changed to same

I hope i am clear with my problem.