All you need is a tent!

Follow the steps down below to do the dupe!
1.Pitch the tent
2.Open up the tent
3.Put all your desired duping items in the tent.
4.Once you have done that Do ALt+F4(Don't close out of the tent, if you leave the tent and alt+F4 it wont work).
5. Rejoin the server you pitched the tent on and you should have the items in the tent and in your inventory!

Another tip if your ammo is low and you need to refill it, put the ammo in your backpack close your backpack and disconnect from the server. Once you rejoin take the ammo out of your backpack and it will be all refilled!

I hope you enjoy this dupe, have fun!