http://simswiki.info/wiki.php?title...al_Site_Archive has a large collection of items that were made available by Maxis through the Sims 2 website. But I already had some of these items before I downloaded them from simswiki. Were some of them released elsewhere?

Some of the released items were in reality just items from the first Holiday pack, or already included in the game, which could explain why you already had those (can't remember exactly which ones, but definitely the bunny, cowplant, halloween items, and potentially the posters + dog house and chair, likely a few more)

Are there any in particular you're missing? There were also TS2 items from the Store + pre-release items, but those were not among the free items. They're available, though.

I'm not missing anything, I'm wondering which ones were released elsewhere. What you said was released elsewhere matches my experiences, thanks.

Oh, and is it safe to remove the duplicate items' packages?

Yeah, totally fine. But remove the packages; don't delete them in game or in Bodyshop.

And it won't delete placed examples of the duplicate objects?

Not as long as they're based on the same objects and have the same GUID (ID number) in the file, then no - placed duplicates won't be deleted.

If they're seen by the game as a new object (if you have two of the same item in the catalog), then it's possible the duplicates will be deleted, but in that case you can remove the old ones (if you recognize them/they have a star) and place the original ones from the catalog before removing the files from your Downloads folder.