^ Just to avoid confusion, it's perhaps a good idea to point out that there's a difference between the Lot Catalog (maxis lots + downloaded lots that show up the ingame catalog, shared between all hoods) and the Neighborhood "Lots" folders (lots already placed in hoods).

The ones in the Lot Catalog are template lots and are generally safe to remove (one exception might be the maxis occupied lots from Pets, not sure about those), but the ones in the "Lots" folders in the Neighborhood folders are used by the hoods, and removing those could cause unwanted results.

I've used Clean Installer to find and delete duplicates, because it highlights them in pink.

Yes, @simmer2z - I should have said that.
Being a builder does mean there are often lots I have builr myself in my bin, and they don't all need to.stay there.

If your duplicates are in order in a list in Clean Installer you can shift + click and remove them all at once.

I've used SimPE for finding duplicates & GUID conflicts. Found issues I didn't even realize my Download folder had (we're talking stuff that had lurked around since 2007-ish. I guess a cleanup was needed...)

Most plain duplicates (if they're identical copies - plain meshes/recolors and such) don't tend to cause issues. If there are differences between the files, it's usually the last read file that decides what shows up. Mods are a somewhat different can of worms, though (working with load order and all that).