Naming Master/Detail pages is automatic. The main page will be named after master table and it will be included in navigation. The Add/Edit pages are named simlar to CRUD template and will not be included in navigation.
Maybe it would be better to ask for a new name for the page that will be created... or ar least give a warning if a page with same name already exists.
Has there been a solution to this, I'm having the same problem.I don't want to delete the original page to add a master detail page from the same Schema.
Noticed this as well but what I found if I change my old pages name "_old" in the page properties I would would be able to generate the new master detail page without any more problems.