Originally Posted by
potado
Super simple.
Send a trade request to a friend. Add the item you wish to dupe. Confirm the trade, and both players simultaneously remove the item from the slot (Press E + R at the same time).
Dupe complete, both players now have the item. You might need some lag, not a 100% success rate but still fairly high.
This has been in the game since launch, the GF and I have exploited it a good bit, but are quitting the game now. The game kinda sucks.
EDIT: elaborated instructions