Which aircraft have fought against its own type in active combat? The criteria for this question are 


The aircraft has to be the same type (but not necessarily the same mark)
The two opposing sides are actively hostile; but not restricted to nation states (so civil war a possibility)
The aircraft treat each other as an active threat 
Cloned aircraft count, e.g. Tu-4 / B-29 or J8M / Me163 (neither of these occurred as far as I know)


It's difficult to see circumstances in which it would happen; but in over 100 years of air combat I think it likely to have occurred at some point.
Probably not the only situation, but in 1948 Israeli Spitfires faced Egyptian Spitfires, and just before the truce also faced a few RAF Spitfires that were too close to the combat area (reference)