Problem

1. I just met myself in town
2. When coming back from vacation, my sim has doubled

Cause

1. This has been around since the beginning. With LN it seems to get worse for there are so many new NPCs in town. What the game does when it creates a new Sim is, that it sometimes just uses a Sim that is in the library. If you have a copy of your Sim in the library, this will lead to a clone of yourself. The two Sims (sometimes more) have no connection to each other whatsoever.
2. There is another clone issue that has been around since WA. That clone is different and is actually the same Sim as yours. They are connected and deleting them will result in corruption of your own Sim. The Gen Patch made this problem a whole lot more common, leaving to all kinds of problems.

Solution

1. To avoid this, just empty out your library. You can export the Sims you want to keep and save the Sims3pack on an external disk. You can safely delete the Sim you don’t want.
2. None

Mods that help with the issue

1. —
2. Awesome Mod, Overwatch V28 or greater.