Sims 4 Game merged 2 save files on its own.

@MysticalLex  I've never heard of this happening before.  Are you absolutely sure you didn't open the old save at some point?  I'd also like to see a screenshot of the contents of your saves folder, including the modified dates, just for reference.

As for prevention, I have no idea how to stop this from happening again since I don't know how it would happen in the first place.  But it's always a good idea to keep backup copies of your saves.  I keep the last three or so sets, actually the entire user folder, and delete the oldest versions when I know I won't need them.

Speaking of backups, you can try recovering a backup copy of your most recent save.  Perhaps the changes won't have been imposed on the earlier versions.

https://sims4.crinrict.com/eng/2014/09/recover-backup-save-game/