In What Happened to Monday, there're checkpoints in where authorities will verify the identity of each individual by scanning a bracelet that the person wears, and then double-checking their face with the picture on file.
All throughout the movie,
the sisters were able to cheat this system simply by making sure only one of them were out in public at a time, so that their bracelets aren't scanned at two different places at the same (or near same) time.
With twins/triplets/etc. not being a foreign idea, wouldn't it have been much better to do a fingerprint scan as well? Given the advanced technology in the film, and the fact that there was already a considerable amount of bio-metric scanning (ex. guns being fingerprint locked, buildings requiring retinal scanning), it seems like this would definitely be possible, and would also offer increased security (since the bracelets seem to have several vulnerabilities).
I feel like this realization makes the movie fairly trivial, since the implementation of such a system would have prevented all but one of the sisters from being able to leave the house, go to school, work, etc..