Note: This trick only works in an Open Battlenet game, as the characters need to be saved on your computer. This trick also requires two computers with two copies of Diablo 2. Get the items to be duplicated. Copy the "Save" folder from the Diablo 2 folder of that computer and paste it on your desktop. Go online with both computers and meet in a game. Give the items to be duplicated to the second computer, then save and exit. Press [Alt] + Tab] on the first computer to switch to Windows and copy the "Save" folder from your desktop back to its original location, overwriting its current contents. Return and join the other computer in the game. You will now have all your original items and the other computer will have a copy of them. Note: This does not work with the +1 Skill book from Act 2, as you are only allowed to do it once.
Note: This trick also only works in an Open Battlenet game. Take the item to be duplicated and place it in your inventory. Then, save and exit the game. Next, come back to the game or join a different one. Drop the item you want to duplicate. Make a note of the game name and password, then sign off of your ISP -- do not save and quit the game. Have another person in the game keep it open while you are logged off. Sign back on to your ISP, go back into the game, and pick the item that you dropped. It will also still be in your inventory, so you now have two copies of that item.