Identical girls, one is bad. But how can an ID be had? The vamp's negligee gives her badness away, while the good girl's more modestly clad.
The Guilty (John Reinhardt, 1947), starring Bonita Granville and Don Castle. Based on a novelette by Cornell Woolrich, this modest Monogram mystery has Granville as twins, though the entire twins bit was added for the film. David first posted this as a comment on an earlier rhyme on the same film,Double for Nothing.