2 ways. Split screen and xbox live/psn.

SS: 1. Sing in to your main account and start playing.
2. Sign into another account (NOT a guest)
3. Give the second account the item you wish to dupe
4. Save and quit
5. Trade the item from the second player back to the first.
6. Sing out of the second account then re join.
You can repeat steps 5-6 as many times as you want.
You can also dupe money

XBL/PSN: 1. Start a game online
2. Invite a friend
3. Give the item to the friend
4. Friend saves and quits
5. Friend rejoins and gives you the item
6. Friend goes to dashboard
7. Friend gives you a duplicate of the item.
Repeat 5-6 as much as you want.
You can also do this with money.

good shit bro! that helped me out a ton with this lvl 50 rocket i needed to dup

thanks bro! that helped me out a ton with this lvl 50 rocket i needed to dup