It is rare to see two different actors win an Oscar for playing the same role. I can only think of one—Marlon Brando and Robert De Niro as Vito Corleone in The Godfather and The Godfather II. John Wayne and Jeff Bridges were both nominated for playing Rooster Cogburn in two different True Grit adaptations, but only Wayne won.
This year, we could very well see a repeat character in Joker, which is somewhat surprising given the blockbuster films Joaquin Phoenix and Heath Ledger were a part of are not the usual suspects for the Academy Awards.