From the AFI:
"When a prominent physician is found stabbed to death, police detective Stevenson's investigation leads him directly to magazine store clerk Terry Collins, who works in the doctor's office building and is identified by co-workers as the murdered man's girlfriend. Stevenson is puzzled by the conflicting descriptions of Terry he hears from people with whom she works, even that of another doctor, Scott Elliott. When Stevenson goes to Terry's apartment, she introduces him to her identical twin sister Ruth, and he understands the witness' discrepancies. The sisters admit that they often fill in for each other at the magazine stand, but when neither will confirm her whereabouts the night before, Stevenson arrests both."
My biggest problem with this film is that it sets up Detective Stevenson as the main character, positioning itself as a mystery, instead of the thriller it becomes. It really is almost two inter-related films, the first third a detective mystery with requisite comic relief and following of clues, and the last two thirds a psychological thriller about twins and the (recurring) old wives' tale that states one of a set of twins must be defective. But this is a complaint about narrative structure and not an indictment against its status as a film noir. In fact, it's not the only film noir that has used a "straight" to give us access to that underbelly of crime (see: KNOCK ON ANY DOOR). And once we get to the psychological thriller, the film is much better. Olivia de Havilland is quite good in her dual role, and I had a hard time (at least in the print I watched) picking up on the line in the process shots. Stevenson returns at the end of the film to help with the denouement, but it's mostly deHavilland's show, and she carries it. The film doesn't have an overabundance of shadow, but mirrors are used very effectively. One thing detracting from the noir atmosphere is the score, which is standard 1940s Hollywood (heavy on the strings) and used far too much, especially after hearing David Raksin talk about scoring LAURA.