The other day at lunch my workmates and I spent an inordinate amount of time discussing people who played multiple roles in the same film. The major one I thought of was Peter Sellers — and Eddie Murphy is practically the canonical example from Coming to America. It vexed me that I could not think of very many off the top of my head, but it was sort of a pleasant kind of ignorance riffling through my mental rollodex, even if it came up empty.
As predicted, there turned out to be an extensive Wikipedia page dedicated to this topic: List of actors who have played multiple roles in the same film.
I suspect it’s incomplete, and if I come up with more entries you betcha I’ll be adding to that page. Perhaps you can think of some?