Prerequisites
* A public container such as a barrel, wardrobe, vehicle or tent.
* A competent friend.

Tutorial

1. Put an empty backpack of your choice into the container, the bigger the better.
2. You start to eat and then put the backpack into your hands, cancel eating right after.
3. You then tell your friend to put the backpack ON HIS BACK and then into his hands.
4. Once your animation for eating finishes, you should both have the backpack in your hands.
5. You both can now load up the bag with whatever the hell you want to dupe.
6. The person that ate the food must throw his backpack away with G.
7. The other person now right clicks the backpack in his hands and removes it from his hands to put it on his back, creating two copies.
8. The person with two backpacks relogs, and boom. Profit.

Enjoy!

-Edit: You can also use this same exact method to dupe a gun. Duping guns with this method include all attachments unlike previous gun dupes.