I just did a test trying to set up a site with the name of one that was already on the Multisite. The system from the admin side doesn’t allow it, but it doesn’t tell the user that their selection is not available.
The same happens with a sub-domain that has gone through the WHMCS/WordPress Provisioning system. There is no message to say that the username has been taken, please select another.
Is there a way to do that?