Sessiona Aynı ürünü iki Kere Ekleme

$(".item_form").submit(function(e){
        var form_data = $(this).serialize();

        $.ajax({
            url: "Kart",
            type: "POST",
            dataType:"json",
            data: form_data
        }).done(function(data){ 
            var totalItemInCart = $("#items_in_shopping_cart, #items_in_shopping_cart_mobile").html(data.items_in_cart); 
            var new_item_qty = $("#items_in_shopping_cart, #items_in_shopping_cart_mobile").html(data.all_items); 
            
            $('#cart_update_info').empty();
            
              swal({
                    title: "ürün eklendit",
                    timer: 1000,
                    showConfirmButton: false
                  });


            if($(".shopping_cart_holder").css("display") == "block"){
                $(".shopping_cart_info").trigger( "click" );  
            }
        })
        e.preventDefault();
    });

if(isset($_POST["item_id"]) && isset($_POST['item_qty']) ){

	$added_item["item_id"]=$_POST['item_id'];
    $added_item["item_qty"]=$_POST['item_qty'];
	$added_item["item_size"]=$_POST['item_size'];
    $added_item["item_color"]=$_POST['item_color'];

/*ajax dan gelen postlari aliyorum*/

$items = $database->find_by_query("SELECT * FROM shopping_items WHERE item_id='{$added_item["item_id"]}' LIMIT 1");

/*sorgudan geciriyorum*/

foreach($items as $item){

                $added_item["item_name"] = $item['item_name'];
                $added_item["item_price"] = $item['item_price'];
                $added_item["item_image"] = $item['item_image'];
                $added_item["sub_item_name"] = $item['sub_item_name'];
                $added_item["link"] = $item['link'];

                $_SESSION["items"][$added_item['item_id']] = $added_item;
                
            }

    /* sepetteki urunu hesaplayip json formatinda cikti aliyorum */
    exit(json_encode(array('items_in_cart'=>count($_SESSION['items']))));

}