Error and MySQL: [ngRepeat: dupes] Duplicates in a repeater are

I am developing an example with IONIC but it is giving the error:

ionic.bundle.js:19387 Error: [ngRepeat:dupes] Duplicates in a repeater are not allowed. Use 'track by' expression to specify unique keys. Repeater: usuario in usuarios, Duplicate key: string:c, Duplicate value: c
http://errors.angularjs.org/1.3.6/ngRepeat/dupes?p0=usuario%20in%20usuarios&p1=string%3Ac&p2=c
    at ionic.bundle.js:7888
    at ngRepeatAction (ionic.bundle.js:32262)
    at Object.$watchCollectionAction [as fn] (ionic.bundle.js:21887)
    at Scope.$digest (ionic.bundle.js:22020)
    at Scope.$apply (ionic.bundle.js:22282)
    at done (ionic.bundle.js:17439)
    at completeRequest (ionic.bundle.js:17629)
    at XMLHttpRequest.requestLoaded (ionic.bundle.js:17570)(anonymous function) @ ionic.bundle.js:19387(anonymous function) @ ionic.bundle.js:16350Scope.$digest @ ionic.bundle.js:22038Scope.$apply @ ionic.bundle.js:22282done @ ionic.bundle.js:17439completeRequest @ ionic.bundle.js:17629requestLoaded @ ionic.bundle.js:17570

I have reviewed the code several times, I have tried to change in ng-repeat with other variables, but either the error does or does not show the data. What can it be?

My PHP:

    <?php

header("Access-Control-Allow-Origin: *");
header("Content-Type: text/html; charset=utf-8");

$data = file_get_contents("php://input");
$objData = json_decode($data);

$dns = "mysql:host=mysql70.ramosdainformatica.com.br;dbname=ramosdainforma70";
$user = "usuario";
$pass = "senha";


$counter = $objData->counter;
$token = $objData->token;

try {   
    $con = new PDO($dns, $user, $pass);

    echo "Abriu con";

    if(!$con){
        echo "Não foi possivel conectar com Banco de Dados!";
    }
    if ($token === "1f3d2gs3f2fg3as2fdg3re2t1we46er45" && isset($token)) {  


        $query = $con->prepare('SELECT * FROM usuario_app ORDER BY nome LIMIT '.$counter.', 5');
        $query->execute();


        $out = "[";
        while($result = $query->fetch()){
            if ($out != "[") {
                $out .= ",";
            }
            $out .= '{"cod_user: "'.$result["cod_user"].'",';
            $out .= '"nome": "'.$result["nome"].'",';
            $out .= '"pais": "'.$result["pais"].'",';
            $out .= '"cep": "'.$result["cep"].'"';
            $out .= '"logradouro": "'.$result["logradouro"].'",';
            $out .= '"complemento": "'.$result["complemento"].'",';
            $out .= '"bairro": "'.$result["bairro"].'",';
            $out .= '"cidade": "'.$result["cidade"].'"';
            $out .= '"estado": "'.$result["estado"].'",';
            $out .= '"celular": "'.$result["celular"].'",';
            $out .= '"email": "'.$result["email"].'",';
            $out .= '"senha": "'.$result["senha"].'"';
            $out .= '"data_cadastro": "'.$result["data_cadastro"].'",';
            $out .= '"latitude": "'.$result["latitude"].'",';
            $out .= '"longitude": "'.$result["longitude"].'"}';            

        }
        $out .= "]";
        echo $out; 


    }
} catch (Exception $e) {
    echo "Erro: ". $e->getMessage();
};

My Controller:

    (function() {
    "use strict";

    angular.module("myApp").controller("initCtrl", function($scope, Data){
        $scope.home = "Usuários";
        $scope.perfil = "Perfil";

        $scope.usuarios = [];

        var getData = function(){
            var params = {
                counter:$scope.usuarios.length,
                token:"1f3d2gs3f2fg3as2fdg3re2t1we46er45",
            };

            Data.getData(params).success(function(data) {
                    $scope.usuarios = data;
            }).error(function(data) {
                console.log(data? data: "Não foi possível acessar ao banco de dados.");
            });


        };

        getData();



});
})();

My Service:

    (function(){
    "use strict";

    angular.module("myApp").value("Config", {

        getUrl: "http://www.ramosdainformatica.com.br/food/"
    });

    angular.module("myApp").service("Data", function($http, Config){
        //recuperação de dados
        this.getData = function(params){
            return $http({
                method: "POST",
                url: Config.getUrl + "apinhac.php",
                data: params,
                headers : {
                                'Content-Type' : 'application/x-www-form-urlencoded; charset=UTF-8'
                            }
            });
        };


    });
})();

My HTML list:

<ion-view view-title="{{home}}">        

    <ion-content>
        <h1>{{home}}</h1>

        <ion-list ng-repeat="usuario in usuarios">

            <ion-item>
                <h1 class="assertive">{{usuario.nome}}</h1>
                <p>{{usuario.email}}</p>
            </ion-item>

        </ion-list>

    </ion-content>
    </ion-view>

I have reviewed the code several times, I have tried to change in ng-repeat with other variables, but either the error does or does not show the data. What can it be?