Not my method, don't bug me about it.

For months certain websites have been using the "Merc Aura Stack Glitch" to dupe. Rather than explaining why the method works, I will just explain how to use the glitch to dupe.

What you'll need:

A character with access to a decent leveled merc
Two items that provide the same aura (ie: HoJ + Dragon on an act one merc, or two dreams on an act three merc)
Access to at least threee Diablo II windows (or two friends)
AutoIt or a packet sender (optional)

How to do it:

1) Window A: Create a private game with the character that has a merc, two items that give the same aura, and items to dupe.
2) Window B: Have a mule join the same game.
3) Window A: Equip the merc with both of the aura items. Type "/fps" in game to get an idea of ping.
4) Window A: Rapidly remove/equip one of the items that gives the merc an aura. Don't stop until you are on step 9.
5) Wait until ping consistently jumps over 4000.
6) Window C: Log on to the character with the items to dupe
7) Window C: Attempt to join the game that Window A and Window B are in, but dont enter a password.
8) Window C: Repeat step 7 until you get the response "Game does not Exist"
9) Window C: Attempt to create a game. If the creation fails, try again (could take up to 10 minutes before it works)
10) Window A: Remove both items from the merc, wait for the lag to stop.
11) Window A: Drop items to dupe
12) Window B: Pick up the items and leave the game
13) Window A: Leave the game
14) Window C: Leave the game
15) Items will be on both the mule and the character with the merc

It can take anywhere from 20 to 120 minutes to cause the game to desynch with one character. If you wait too long it is possible that the game will crash.

To make things easier, you can use either AutoIt or a packet sender. With a packet sender you'll want to spam the 0x61 (equip/unequip) packet at intervals varying from 50-1000ms.