The Dark Mirror (1946), directed by Robert Siodmak.
A minor murder mystery about twin sisters, one sweet and the other a psycho-killer who can feign sweetness. How to discover who is who?
It's thick with the sort of psychiatric plot so popular in the middle of the 20th century; see Spellbound (1945).
Olivia de Havilland gets to play two roles: sisters with distinct personalities who have practiced pretending to be one person. The film uses classic twin effects; some of the rear projection is pretty good.
Dimitri Tiomkin score.
Olive Films Blu-ray. The opening reel looks about like DVD quality but it gets better. Only a few scenes are "good" in terms of image quality.