Tatiana Maslany said in an interview that she has a continuity team that keeps her from mixing up the characters. For instance, if she's doing a scene that has both Alison and Sarah in it and she's playing Alison, if the script tells her to roll her eyes, they make sure it's an 'Alison' eye roll and not a 'Sarah' eye roll.

Cosima mentions in season 1 that Katja's fingerprints would be close enough to Sarah's that running Katja's prints through standard police search would pull up Sarah's rap-sheet. No one seems worried that Beth's fingerprints should have been just as similar, and as a police officer her prints would definitely be in the system. Why didn't Sarah's rap-sheet emerge when Beth applied to become a police officer? Why did the system match Katja to Sarah but not to Beth? The system may have matched Sarah to Beth, but other data would not have matched, such as date of birth, place of birth, parents etc. The match would have been seen as a false positive which often occurs in fingerprint matching systems. Even in ten-print matches, depending on what level of search was employed. In many systems, technician intervention is required to eliminate candidate matches.

Featured in Chelsea Lately: Episode #7.115 (2013)