Hi, not sure if this is the entirely right place to be posting this, but I thought I'd try. I just wanted to check a couple things to ensure that the actions I'm taking aren't going to corrupt my save by accident.
So I moved from Moonlight Falls to Sunset Valley using Traveler. I downloaded the house I lived in and reuploaded it in Sunset Valley (because I really like my house). Sadly, I lost all my custom paintings while doing this. So I went back to Moonlight Falls using traveler and I put all the paintings from the copy of the house that was still there and put them in my sim's inventory before going back to Sunset Valley with them. Sadly, my game performance isn't great and my computer could be better, so it took a very long to do this thanks to the loading screens and my game save increased significantly in size both times I traveled from/to Moonlight Falls. So when I discovered I was missing things like my perfect flame fruits from my fridge or my collection of jewels from my alchemy cabinet (both emptied by the move), I was hesitant to use traveler to go back again. So instead, I had what I hoped was the bright idea to go into a previous save file I had of that game from before I moved to Sunset Valley, make a random unimportant sim, fill their inventory with everything I wanted from my old house that I lost in the move. This included my vegetables (to be fair, it takes a very long time to grow all perfect quality tomatoes, apples, and, uh, steak), all my alchemy supplies (my jewels, my insects, my perfect quality alchemy garden ingredients), more custom paintings and a lot of photos my sim took (both of which have my sims' names attached to them). Then, I went to edit town, saved only that sim to my library, exited the game without saving, went back to my current file, went to my library, downloaded the sim and merged it with my current household, took all the stuff from her inventory, TA'd her, and it was done (in retrospect, this sounds a little cruel to the sim, but anyway). And it seems fine so far, everything is working, but I haven't heard of anyone doing this before (moving items in between save files) and I just wanted to make sure that I'm not going to break my game by doing this, especially considering now I have duplicates of many of these objects thanks to them still existing in MF in my current save.

There's nothing wrong with transferring content via the extra sim you used. In fact, it's likely easier on your save than if you'd sent your own sim back to Moonlight Falls or even if you'd remembered to pack the items the first time around: traveling with a very full inventory can add a meaningful amount of stress to the game engine. I'd wonder whether the photos still had all their data, but you'd know if they lost anything just by looking at them.

Custom paintings and photos do increase the size of a save, and at some point, saves can get large enough that they need cleaned. (Look up Kuree Save Cleaner if you're curious.) So bringing the items to Sunset Valley means you'll probably have to do that sooner, although maybe you won't play the save long enough for it to matter. It's not a big deal to clean the save though, and needing to do so doesn't mean the save itself is corrupt in any way.

The duplicate objects don't matter other than affecting the save file size: since only one world gets simulated at a time, the objects in the other world don't have any effect.

So the short answer is no, you probably didn't corrupt your save. I'd still keep a backup from before your sim moved, just in case, but if you're not seeing any issues from the move now, you probably won't later on either.