Hi Guys
I didn't find this myself. I can't even remember where i first seen it. But here is a rundown on a working dupe on multiplayer servers, it's exploiting a little bug in the iconomy addon.
Dupe method:-
1. Setup a chest shop. If you don't know how check here or on your servers forum. You'll need to make sure the item you are selling on the sign isn't actually in the chest or it will cause it to bug and not work sometimes.
2. Stand at the chest ready to use it and type a command to warp/spawn (Note, you have to warp to somewhere on the same map, if you warp to another map or cross server it will not work. you also need to warp a fair way away from the chest, i haven't checked actual distance but i would go at lest 1000 blocks away, and finally no one else can be near the chest).
3 After you have typed the warp command and are waiting to teleport click the shop sign (with an empty hand, sometime items in your hand can bug it) as if you are buying, the chest should open you'll be able to see & interact with the chest inventory.
4. Now wait until you have teleported (don't interact with the chest contents until the teleport is complete). Now move items for the chest into your inventory.
5. Go back to your chest shop to check for success, anything you moved after the warp should now be in the chest & your inventory.
Troubleshooting:-
So far the things i have noticed that shopped it from working.
If the item on the sign is in the chest it will sometimes stop working, if it stops for an extended time just break & replace the sign.
If you don't warp far enough away, i go at lest 1000 blocks away now.
If someone else is near the chest it will not work (I once had a mod near me disguised as an animal and it took me ages to work out why it stopped working). I think this and warping to close are the key to how this works, I think the server believes if no one is near the chest when the items are removed then the items couldn't have been touched.
If you have something in your hand when you click the sign sometime it'll bug out.
If you warp to a different map this will not work, the easiest way to tell if you haven't warped on the same map is during the warp your game client will go black for half a second and the open chest interface will disappear, if this happens you need to pick somewhere else to warp to. If you play on a server with the inability to warp too two different places on the same map then this will now work. This is the only restriction that could stop this working on a server.
Tip:-
You'll run out of space real fast, dupe a fu*k load of chest first.
Selma