Duplicate Books (i.e. same author and title) are allowable. Try this:
- create another 'book record' via the Add Books->Add empty book->Duplicate current book feature.
- drag (it will copy) the format file you want from 'original' book folder (press 'O' to open it) into the just created 'empty book' Book Details panel.
- remove (with calibre) the format file just copied from the 'original' book.
BR