code: public function checkUniqueRecipient($varValue, Contao\DataContainer $dc)
  	{
  		$objRecipient = $this->Database->prepare("SELECT COUNT(*) AS count FROM tl_newsletter_recipients WHERE email=? AND pid=(SELECT pid FROM tl_newsletter_recipients WHERE id=?) AND id!=?")
  									   ->execute($varValue, $dc->id, $dc->id);
  
  		if ($objRecipient->count > 0)
  		{
  			throw new Exception(sprintf($GLOBALS['TL_LANG']['ERR']['unique'], $GLOBALS['TL_LANG'][$dc->table][$dc->field][0]));
  		}
  
  		return $varValue;
  	}
docs: Check if recipients are unique per channel
  
  @param mixed                $varValue
  @param Contao\DataContainer $dc
  
  @return mixed
  
  @throws Exception