Unique Coupon

function check_coupon(){
	// restricted usage numbers
	$user=&JFactory::getUser();
	$db	=& JFactory::getDBO();
	$coupon = JRequest::getString('coupon','');
	if(!empty($coupon)){
		$db->setQuery("SELECT COUNT(order_id) FROM #__hikashop_order AS o".
		" LEFT JOIN #__hikashop_user AS u ON o.order_user_id=u.user_id".
		" WHERE u.user_cms_id=".$user->id.
		" AND o.order_discount_code='".$coupon."'"
		);	
		$already = $db->loadResult();
		// the number 1 below might be replaced with a parameter from the backend
		if ($already>1){	
			$app =& JFactory::getApplication();
			$app->enqueueMessage(JText::_('COUPON_ALLREADY_USED'));
			return false;
		} else {
			return true ;			
		}
	} else {
		return true;
	}
}