[jkaplowitz]

It was common in NYC even a couple of years ago. Very likely it was a bug indeed, but unless there's a good technical reason why such bugs would only happen with merchants who can easily be reached to arrange refunds and who will routinely cooperate with those refunds, unwarranted double charges should be disputable via chargeback. (Naturally, not all double charges are fake - some are legitimately paying the same amount more than once. Being able to start a chargeback procedure doesn't prevent the merchant from showing the bank evidence that the charges were correct.)