UserTable::onBeforeAdd

static function onBeforeAdd(Event $event)
{
	$result = new MainORMEventResult();
	/** @var array $data */
	$data = $event->getParameter("fields");
	if ($res = UserTable::getList([
		"select" => ["ID"],
		"filter" => ["USER_ID" => $data["USER_ID"]]
	])->fetch())
	{
		$result->addError(new EntityError("Error: User is already exists.", "event"));
		return $result;
	}

	return self::modifyData($event, $result);
}