Добавление только одного товара в корзину

global $APPLICATION; 
$APPLICATION->ThrowException('your message'); 
return false;

\Bitrix\Main\EventManager::getInstance()->addEventHandler('sale', 'OnBeforeBasketAdd', 'ddOnBeforeBasketAdd1');
function ddOnBeforeBasketAdd1(\Bitrix\Main\Event $event) 
{ 
   $arBasketItems = array();
   $dbBasketItems = CSaleBasket::GetList(
           array(
                   "NAME" => "ASC",
                   "ID" => "ASC"
               ),
           array(
                   "FUSER_ID" => CSaleBasket::GetBasketUserID(),
                   "LID" => SITE_ID,
                   "ORDER_ID" => "NULL"
               ),
           false,
           false,
           array("ID", "CALLBACK_FUNC", "MODULE", 
                 "PRODUCT_ID", "QUANTITY", "DELAY", 
                 "CAN_BUY", "PRICE", "WEIGHT")
       );

   while ($arItems = $dbBasketItems->Fetch())
   {
       $arBasketItems[] = $arItems["PRODUCT_ID"];
   }
   if(in_array($aFields["PRODUCT_ID"],$arBasketItems))
   {
      return new \Bitrix\Main\EventResult(\Bitrix\Main\EventResult::ERROR, new \Bitrix\Sale\ResultError('Вы уже добавили данный урок в корзину', 'code'), 'sale');
   }
}