There is very easy way to duplicate items using Storage blocks and 2 players:
1. Player 1 opens chest interface
2. Player 2 destroys chest and picks up all items
3. Player 1 still sees items inside chest interface and can move them to its own inventory
So then both Player 1 and Player 2 have items from chest in inventory.
1. Player 1 opens chest interface
2. Player 2 destroys chest and picks up all items
3. Player 1 still sees items inside chest interface and can move them to its own inventory
So then both Player 1 and Player 2 have items from chest in inventory.