I've had a request for support from someone trying to register a new site but when they try they get the (known to us) message that they are trying to register a site that had already been registered using a different URL. The normal procedure is to click Register a new site -but that did not work for them.
I have been trying to troubleshoot this and I found that they create new sites using a template where they copy the database as well as the Moodle folder. I dont know if tha tis significant but I am linking to a previous, related, tracker issue.
I will add more info in the comments below, restricted.