Once upon a merry time ago I was the first kid on my block to beat Diablo 2 with a Paladin. One thing that I hated was farming for runes and gems.
We discovered pretty quickly how to use multiplayer mode and a back up of the player file to dupe gems and runes so we could create things like our triple socketed perfect diamond great shield in a lot less than the forever that farming offered.
Has this bug been patched over in the latest patches? (I will follow up with a question about the best way to farm runes/gems that I can use on battle.net)
There is no reason to dupe in Diablo 2 on multiplayer LAN or open battlenet since the characters are saved on your local hard drive and you can download an diablo 2 character editor (there are many) to give yourself whatever you want, even change attributes on items to make them super overpowered.
As for closed battlenet, duping still exists, in particular, high end runes are still often duped on the ladder where they can be used in powerful ladder only rune words. The reason why runes are the choice for duping is because they cannot ‘poof’ if they are immediately used in an item for a runeword as they take on the UID of the item they are put in, but if two duped items with the same UID enters a game, they will dissapear.