I havent confirmed this, but on a server im playing on 3 ppl r presently using a glitch/big or what ever you call it and they explained to do it like this:

1. Load up 2 clients and get them onto the Multiplayer Screen using the same account

2. Join Client A to a server and drop what ya want to be copied/Duplicated and exit

3. join Client B quickly to the same server and pickup dropped items and you now supposedly have dble what you started with.