Instructions
Right-click on your partner and initiate an item transfer.
Select the item you wish to dupe from your inventory.
Click "Accept" and trade the item to your partner for nothing.
Click "Accept" then "Decline" in rapid succession. This will cause the item transfer window to close on your screen, but not on your partner's screen.
Right-click your partner and attempt to trade with him three times.
Have your partner accept the trade then log out of the server. Both you and your partner will have the items you attempted to trade in your inventories.