I am not responsible for anything that happens to your account OR your save data/items. Use at your OWN risk.
1) Create a new account on your PS3 (Doesn't need online) or find an old one that does not have any Terraria data on it.
2) Copy the Terraria data you want to duplicate items from to the new/unused account.
3) Play Terraria on the account you just copied the data to.
4) The game will tell you that you cannot save data on this new account because the data belongs to a different account.
5) Enter the world you wany to copy items from on the new account.
6) Take a 2nd controller and log into this world with the character from your main account or whichever character you want the duped items on.
7) Take any items from chests that you wish to duplicate.
8) Have the 2nd player leave so that the game is no longer split screen, then quit the world without saving.
9) Re-enter the world as many times as you want, as the world will never save, but the 2nd character you log into the world with will keep the items on them.
10) Go back to your main account and drop off any items you took from the non-saving world into your main one. -==-Does not work on 360!-==--==-I tried it and it works EASILY!-==-