Hindi cinema has a "twin" convention which allows an actor/actress to play two roles (for the price of one, as far as the audience is concerned). Usually these two characters are just brothers or sisters--the twin is in the perception of the watcher. I know someone who has been doing some work on this subject, but I have no references in print. However, it's interesting to watch movies like SEETA AND GEETA, in which each sister complements the other. Carol Slingo