code: 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;
  	}
docs: Make sure there is only one fallback per domain (thanks to Andreas Schempp)
  
  @param mixed                $varValue
  @param Contao\DataContainer $dc
  
  @return mixed
  
  @throws Exception