What are we counting as 'unique'? Because I'd say that the difference between the three entries in the Street Fighter 4, and the fact you had to pay for each makes them unique, is little more than character additions and updates. And that's fine. But I doubt you'll find very many competitions for Street Fighter 4 prime. Smash already changes a good amount each game without changing so much so as to be convincingly a new series. It gives moderate graphics boost, a bunch more levels (in a game where the levels actually matter), more chars, items, modes.
I can't explain MK3, though, that's just awful.