There used to be a way to dupe an item if (I think) you traded it in steam trade and in game at the same time, which would sometimes produce two of the item, that's why they disabled in-game trading.
How to tell if it's duped: read through this thread. http://www.skial.com/threads/how-to-spot-duped-items.14011/
Basically, if there's another one with the same original id, they're duped
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