core-bundle/src/Resources/contao/dca/tl_page.php
public function checkFallback($varValue, Contao\DataContainer $dc)
	{
		if ($varValue == '')
		{
			return '';
		}

		$objPage = $this->Database->prepare("SELECT id FROM tl_page WHERE type='root' AND fallback=1 AND dns=? AND id!=?")
								  ->execute($dc->activeRecord->dns, $dc->activeRecord->id);

		if ($objPage->numRows)
		{
			throw new Exception($GLOBALS['TL_LANG']['ERR']['multipleFallback']);
		}

		return $varValue;
	}
public function checkFallback($varValue, Contao\DataContainer $dc)
	{
		if ($varValue == '')
		{
			return '';
		}

		$objPage = $this->Database->prepare("SELECT id FROM tl_page WHERE type='root' AND fallback=1 AND dns=? AND id!=?")
								  ->execute($dc->activeRecord->dns, $dc->activeRecord->id);

		if ($objPage->numRows)
		{
			throw new Exception($GLOBALS['TL_LANG']['ERR']['multipleFallback']);
		}

		return $varValue;
	}
Make sure there is only one fallback per domain (thanks to Andreas Schempp)

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

@return mixed

@throws Exception