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