Requires two players.
Both players have no backpack equipped.
One empty backpack is placed on the ground.
Both players look at the backpack and see the hand icon / "Take Bag" action.
Both players execute the "Take Bag" action at the same time.
Both players now have the same backpack equipped. This duped the backpack.
Now one of the players places an item inside the backpack.
The item is now inside of both backpacks.
Both players put the item of their backpack onto the ground. This duped the item.
Repeat.

This method allows to dupe backpacks AND items with the same approach.