Duplicate items
Place the item you want to copy on the ground and walk away from it, keeping it on the screen.
Click on the item. (Your character will walk towards the object.)
Click on any belt item as your character grabs the item on the ground. (You may have picked up the object anyway, but that's okay.)
Open your inventory and put the belt item into an empty space. If done correctly, that item should become the object you wanted to duplicate.

Bypass Switches to Diablo's Room in Multiplayer Mode
Use your telekenisis ability on the two skull levers at Diablo's level. This will unlock the room without having to flip a dizzying number of switches.