Instructions
Duping in Single-Player Mode
Use a town portal item when outside of a town. This will create a portal to the town of your choice.
Move the item of your choice from your inventory and drop it on the ground, next to the portal.
Click the mouse button on your item. Hold the item with your mouse button and use the keyboard to move your character through the portal.
Check your inventory when you make it to the town. You should find that the item you were holding with the mouse button is in your inventory.
Re-enter the portal to return to your original location, and you will find an identical item next to the portal.
Duping in Multi-Player Mode
Fill up your belt with healing and mana potions while playing, and drop any item from your inventory on the ground.
Walk a short distance away from the item, then click on it with your mouse to begin walking back towards the item.
Once you are about to pick up the item, click on one of the potions in your belt.
Drop the potion you just clicked on. When it hits the ground, it should turn into the item you just picked up. If it does not work, repeat the process until you perfect the timing.