Pull to refresh data duplication

I am creating an Ionic application that is pulling articles from a joomla K2 website. I am using $http and just ending my url off with '?format=json' and that is working perfectly. However the website I am pulling data from updates its articles every few minutes so I need a way for the user to be able to refresh the page. I have implemented Ionics pull to refresh and it is working swell except for the fact that instead of just pulling in new articles it just appends all the articles to my array. Is there anyway to just maybe iterate over current articles timestamps or IDs (I am caching articles in localStorage) to just bring in new articles? My factory looks like this:

.factory('Articles', function ($http) {
    var articles = [];
       storageKey = "articles";

    function _getCache() {
        var cache = localStorage.getItem(storageKey );
        if (cache)
            articles = angular.fromJson(cache);
    }


    return {
        all: function () {
            return $http.get("http://jsonp.afeld.me/?url=http://mexamplesite.com/index.php?format=json").then(function (response) {
                articles = response.data.items;
                console.log(response.data.items);
                return articles;
            });
        },

        getNew: function () {
            return $http.get("http://jsonp.afeld.me/?url=http://mexamplesite.com/index.php?format=json").then(function (response) {
                articles = response.data.items;
                return articles;
            });
        },

        get: function (articleId) {
            if (!articles.length) 
                _getCache();
            for (var i = 0; i < articles.length; i++) {
                if (parseInt(articles[i].id) === parseInt(articleId)) {
                    return articles[i];
                }
            }
            return null;
        }
    }
});

and my controller:

.controller('GautengCtrl', function ($scope, $stateParams, $timeout, Articles) {
    $scope.articles = [];
    Articles.all().then(function(data){
        $scope.articles = data;
        window.localStorage.setItem("articles", JSON.stringify(data));
    }, 

    function(err) {
       if(window.localStorage.getItem("articles") !== undefined) {
          $scope.articles = JSON.parse(window.localStorage.getItem("articles"));
        }
    }

    );

    $scope.doRefresh = function() {
    Articles.getNew().then(function(articles){
      $scope.articles = articles.concat($scope.articles);
      $scope.$broadcast('scroll.refreshComplete');
    });
  };
})