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