Blizzard became aware of the issue last night and shut down the auction house. As of midnight Pacific, the company had created a fix for the problem, but it still hasn't been implemented. The studio has decided not to rollback the update, and will instead "make corrections" to individual accounts. Writing on the Diablo 3 forums, community manager Lylirra stated, "We feel that this is the best course of action given the nature of the dupe, how relatively few players used it, and the fact that its effects were fairly limited within the region."
We've contacted Blizzard regarding the current status of the problem.
[Thanks, Pablo and Erik!]