newsletter-bundle/src/Resources/contao/dca/tl_newsletter_recipients.php
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 $varValue;
	}
public function checkUniqueRecipient($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;
	}
Check if recipients are unique per channel

@param mixed                $varValue
@param Contao\DataContainer $dc

@return mixed

@throws Exception