Place the desired item(s) on a character, and ready or create a second character to take on as many of the item as you wish.
Start a two-player co-op game with the first character as player 1 (and the other character as player 2, obviously).
Give desired item(s) to player 2 and have player 2 save and leave game.
Quit game from PS3 system menu.
Because the game doesn't save player 1 when player 2 leaves, nor when you quit from the system menu, now both players have said item(s).
Repeat as desired (for faster duplication, transfer all copies of the item to player one, save-and-quit with both, and start over with the duplicates on player 1).