This is a very simple and fast method to duplicate items on Resident Evil 5.

1) Start any chapter alone.
2) Invite a friend to play with you.
3) When the player joins, have your friend give you the item/weapon he would like to duplicate.
4) Have your friend quit the game and select "Do Not Save".
5) Then you quit the game but save.
6) Load the game and invite your friend back.
7) You should both have the item/weapon that your friend gave to you.
8) Give the weapon/item to your friend.
9) Have your friend quit and save.
10) Leave the game without saving.
11) Load up the level again and repeat the process, however with you receiving the item this time.
12) It should alternate each time you load up the level.
13) Repeat until inventory is full.
14) Now you can sell/keep what you like.