The Dark Mirror (1946) Olivia de Havilland Eng

Lt. Stevenson is in charge of the murder investigation of Dr. Frank Peralta, who was found dead in his apartment by his cleaning lady first thing in the morning, he stabbed in the heart. The chief suspect is the attractive young woman seen leaving his apartment the night before, Stevenson eventually learning she his girlfriend, Terry Collins, who operates a confectionery stand in an office building lobby. When Stevenson questions Terry, she has an airtight alibi in being seen at exactly the same time somewhere else. It isn't until Stevenson visits Terry later that the explanation for that double vision is that there are two of them, namely an identical twin sister named Ruth Collins, roommates who knew this information would come to light. Having planned their stories, they as a combination admit to one being seen out and about, the other at home alone, neither stating who was who with the one out and about protecting the one at home who would not have anyone to corroborate her story. With none of the eyewitnesses able to confirm who they saw between the two sisters, the sisters seem to have pulled off the plan successfully. However, one of Dr. Peralta's colleagues, Dr. Scott Elliott, who often patronizes Terry's stand, believes he can help Stevenson in he actively studying twins as part of his on-going research. Although somewhat suspect, the sisters ultimately agree to be part of Elliott's research, he not telling them about his association with Stevenson. Elliott knows that if one of the sisters truly is the killer, the innocent sister may be at risk. Complications ensue as a mutual attraction develops with both sisters and Elliott in the process, his life also at risk if he doesn't provide an accurate assessment of the situation.