1. get a secure chest
2. break the lock off
3. get it low health enough to break it in 1-2 hits
4. get your friend to put the items inside of it which youd like to dupe
5.you open it or your friend you singularly take each item out but stay inside the chest on your screen when its empty
6.get your friend or yourself whoever isnt on the chest screen to break it
7.once you break it a loot bag from the chest will drop on the ground and your friend will still have the items in his inventory.

The way this works the server doesn't register that the items have been taken out of the chest until you close it so if you break it when the items are inside your friends inventory for some reason it also drops it on the ground, also if you are duping say a whole chest of gold if you drop it on the ground then your friend picks it up that also works