Open you inventory left click the item you want to duplicate. Drag it outside of the inventory screen and (Make sure there is an X on the item) While still holding the left mouse button press Cntrl + L
(Quick load) while the game is quick loading and the load bar starts to fill let go of the mouse button when the game loads your original item will be in your inventory and the duplicate will be on the ground where you put it.
Quick save before you try this and when it is successful quick save again before you try to duplicate another item or they will be lost.

If you have 20 super potions it will duplicate 20 super potions giving you 40 total it works with everything as far as I can tell even money. I noticed with weapons though that when the one your are using receives damage the duplicate does too.
I found this on accident and it took me awhile to realize what I did and a little while longer to figure it all out.