code: public function checkFeedAlias($varValue, Contao\DataContainer $dc)
  	{
  		// No change or empty value
  		if ($varValue == $dc->value || $varValue == '')
  		{
  			return $varValue;
  		}
  
  		$varValue = Contao\StringUtil::standardize($varValue); // see #5096
  
  		$this->import('Contao\Automator', 'Automator');
  		$arrFeeds = $this->Automator->purgeXmlFiles(true);
  
  		// Alias exists
  		if (\in_array($varValue, $arrFeeds))
  		{
  			throw new Exception(sprintf($GLOBALS['TL_LANG']['ERR']['aliasExists'], $varValue));
  		}
  
  		return $varValue;
  	}
docs: Check the sitemap alias
  
  @param mixed                $varValue
  @param Contao\DataContainer $dc
  
  @return mixed
  
  @throws Exception