Been playing with my buddy, got duping to work. To dupe anything, including gold, just hit a checkpoint (cause an auto-save) then enter a trade with your friend who is hosting the game (2nd player required). Put up the gold or/and items you want to dupe. complete the trade. This is where some trial and error could likely slim down the process, but what I do and know works is unplug my flash drive as soon as the trade closes, then dashboard. Reload the game, plug in your drive, and you'll have all your stuff along with the copies your friend has.
Not anything crazy complicated, but figured it was worth sharing.
EDIT: no need to remove the drive, simply dashboard out and reload the game.