Though many Hearthstone players have been able to download and enjoy the Curse of Naxxramas expansion's Plague Quarter, there is also a significant number of users that have experienced issues. Today, Blizzard has issued an update for those that had trouble with their purchase.
Those users that purchased the expansion directly with real money and were double-charged were reimbursed the extra money on Friday, with Blizzard sending out notification emails. Unfortunately, those that attempted to make a purchase through iTunes will have to take the extra step of contacting Apple Customer Support, due to billing limitations.
Meanwhile, those that were double-charged on their in-game gold purchases will need to log out of their accounts and log back in. A hotfix should have been implemented to restore the lost gold. If the excess gold has not been restored, contact Blizzard.