It will create duplicate girls if someone has downloaded, for example, Nena Trinity (with "Nena Trinity.girlsx") from my official packs and then they turn around and download a Gundam 00 pack (with "Gundam 00.girlsx") from someone else. Now their game has 2 Nena Trinitys in it. I've not observed a crash yet, but the game doesn't handle duplicates with the same name correctly at all. Like in the turn summary, if you've ever seen that problem before, the game gets confused about which girl is which.
Since the official packs have separated .girlsx files for every girl, I suggest anyone making custom packs to do the same. You don't have to if you don't want to.