The Dark Mirror

Classic film noir directed by Robert Siodmak. When a man is found with a knife in his back, Lieutenant Stevenson (Thomas Mitchell) is put in charge of the murder enquiry and quickly narrows his list of suspects down to identical twin sisters Terry and Ruth (both played by Olivia de Havilland). Psychologist Scott Elliott (Lew Ayres) is then enlisted to help sort good sister from bad, but his task is soon made more complicated when he falls in love with one of the suspects.