In the article I wrote before Diablo 3 launched I made this prediction about duping.

Now what about duping in Diablo 3? I have heard a lot of people say "Duping will be an impossibility in Diablo 3" or that "World of Warcraft didn't have any dupes". Unfortunately both those points are wrong.. In the beta, various dupe methods have already been found out and quickly fixed. On the topic of WoW, it has seen many exploits over its lifetime that have allowed for items to be copied. Luckily for us it never became a serious problem, due in part to the majority of items being non-tradable and the quick reaction to duped gold and tradable items being removed from the game. The average player never even knew things like this were happening, and I am honestly impressed with how Blizzard handled the situation and stopped it from becoming a problem in WoW.

Duping will exist in Diablo 3; there is no way around that fact. But like hacking it will not be as large of a problem as it sounds. Blizzard has shown with WoW that they have become a modern day Skynet when it comes to monitoring for illegally obtained items. Added to that fact is the real-money auction house, so it is far more important for Blizzard to keep the economy as secure as possible. Copying items is the one thing that could make or break the Diablo 3 auction house, so there is absolutely no way Blizzard will allow that happen.

Unfortunately I was wrong about it not being a large problem. The always online infrastructure does not make dupes impossible, I cannot think of a single game that does not have them. It just makes it a lot harder and easier to fix on Blizzards side. Although to be fair the majority of what I hear is that duping is through account restorations.