Apart from different versions of the same game sharing an IFID (The Apple II and the ZX Spectrum versions of The Sorcerer of Claymorgue Castle are very different, but you can’t have both at the same time in the Spatterlight library) the only other example of note I can think of is the two versions of Zork I and Suspended which both have IFID ZCODE-5-------, which Spatterlight now can handle.
There are probably more out there that I’ve missed. As far as I know it happens by default if you create an entirely new game by modifying the Inform 7 project of an older one.