Let's call it the look-alike plot: Two siblings, one's good and one's not In double-exposure one loses composure One actor acts twice in one shot
Hollywood writers love the look-alike gimmick--and actors love to chew the scenery in dual roles. A subset is the evil twin plot, where usually the bad sibling impersonates the good. Among the actors who've done double-time is Olivia de Havilland in The Dark Mirror (Robert Siodmak, 1946). Top: Den of Geek!; Above:Shooting in the Dark; Below:filmsnoir.net.