It is not recommended to edit PHP files via Dashboard, because when something went wrong the Dashboard will be locked and I can’t be fixed without FTP access.
Meanwhile, if your changes in the header, that was done via the child theme, are not important, just re-name the header.php file via FTP and this file will be taken from the parent theme, with no logo duplication.
Did you made changes by yourself or asked a developer? Try to take a new header.php file from the parent theme into the child and made your changes one more time there?
Regards