The Dark Mirror

Dr. Frank Peralta is stabbed to death in his apartment one night. The detective on the case, Lt. Stevenson (Mitchell), quickly finds multiple witnesses putting Peralta's lover, Terry Collins (de Havilland), at the scene. However, when Stevenson finds Terry and questions her, she has an iron-clad alibi with multiple witnesses. It is revealed that Terry has an identical twin sister, Ruth (de Havilland), and the pair share the same job and routinely switch places for their own benefit. Stevenson and the district attorney are unable to prosecute, since the twins refuse to confirm which one of them has the alibi.