No, it's not my sim, he's one of the residents I made to populate the town. But that's a good point about checking to see if he's a clone. So I did, and he isn't. But once I finished populating the world starter save, I removed all of the resident households from the library. I also have immigration disabled with NRaas Story Progression, and I think that would prevent clones from appearing anyway, wouldn't it?
I thought about the salon, that maybe he could have had a makeover. But when I've had my active sims get a makeover, the new outfit never replaced the original, it was always added as an extra. Could it be different if an inactive sim goes to the salon? But it seems unlikely that he'd get all of his outfits replaced, even if he did get a makeover at the salon, and he doesn't have any of his original outfits.