(Please start new threads for new issues — the thread you posted to isn't related. I've moved this to a new thread.)

A duplicate library would be due to using export/import on one computer in addition to syncing from another. Sort the items list by Date Added and you can likely see the entire set of items you imported. If you haven't used those anywhere (e.g., in a word processor document) or moved them into collections, you can just delete them. If you've done more, it's hard to give general advice.