core-bundle/src/Resources/contao/dca/tl_page.php
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 $varValue;
	}
public function checkFeedAlias($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;
	}
Check the sitemap alias

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

@return mixed

@throws Exception