I've recently placed the same household of 8 sims out of my library and into the same neighborhood 4 times, and played for a sim week with no issues.
I have accidently had two copies of a Sim running around town. I play with Cas.fulleditmode on all the time cause I'm always fixing townies with their odd outfits and purple or green hair. So I just shift clicked the extra sim when I saw him and fixed the name and gave it a makeover. However you can also replace it with a sim from your library or one from the gallery. Easy to do and it's a quick fix.
@ReneHollyDesign1 - Assuming it didn't break my game, this is precisely the solution I had in mind. Thank you. With the answers given by everyone, this gives me some peace of mind. Now I can confidently send Molly back to Sulani with her lovely Vampire Fiancée with no worries. I might just change the Copperdale Molly's name (was considering Milly or something close to maintain some consistency), and my rescued version's wardrobe. Should suffice.
