What's the trick?

Put an item (e.g. a potion) on your belt. Then drop another item (e.g. a stack of coins) on the ground, then walk away a bit. Click on the coins on the ground, then the instant before it gets picked up, click on the potion. If you do it correctly, dropping the potion on the ground will drop a duplicate stack of coins instead.

Tip: an easier way to know if you've duplicated the coins is that when you pick up the potion, the bottom of your screen should say you've picked up coins.