This game is also available on PS3.
You can also do this locally, but you'll need two controller and two profiles. Enter a single-player game with the items you want to duplicate and enough room for the duplicated items. Exit to the game menu. Enter local cooperative mode and login the second player as the character used in Step 1 (the first player should already be this character due to the prior login.) Enter the game. Be sure that your two characters are one and the same. Player two should drop the items to be duplicated while player one picks them up (if done in reverse the items will be lost forever!). Exit to the game menu. The next time you check your character's items there will be two of them.