@caitobr7
I’m guessing you saved the sim to your account before moving them into a lot. You might do that, for example, if you want to upload a copy of the household to the gallery to share with other players or if you’re not ready to make them a playable character yet. So a copy of the naive character exists in your inventory. (As a related aside, a household created by another player that you add to your account from the gallery will be placed in the same inventory.)
Your game spawns a number of NPC Sims to populate your game but which don’t live in any of the lots. These are labeled as ‘other households’ but ‘not in this world’ (i.e. you can’t locate them from the world map). In this case your game sampled from your inventory and placed a copy of that initial save into the game as an NPC => you have a Doppelgänger!
Hope that makes sense, theoretically speaking. And, if so, does it line up with your case?
FWIW: You can remove them from ‘other households’ if you don’t want them in your game. Because I really enjoy random variation, I would probably leave them in my game as an interesting character to do something fun with. Like, make them my Sim’s new BFF...or, more likely (knowing me), their arch nemesis.
🙃