It is possible to duplicate both items and currency in the online game modes. Breaking a stack and re-stacking gems with the vendor sell window open increases the value of the items to sell exponentially. It's possible with 2 of the same gems to earn infinite gold.

Moving items from the player inventory into the player chest before immediately exiting to the main menu to accept a player invite and joining their game caused the items to both be present in the player inventory and the player chest.