I've never been double billed by a restaurant, although I've had experiences where mistakes were made and what was billed did NOT agree with what was on my signed receipt.

I suspect on the rare occasions where double billings take place, its usually an honest mistake. My wife oprates a small, business. She accepts credit cards and despite numerous safeguards, "accidental" double-billing isn't 100% preventable.

I can also assure you that credit card companies rightfully have no tolerance for repeat "double billing" merchants. That's why I don't think anyone who attempted the practice would get away with it for very long. More likely they'd be attempting to explain what went wrong to the police instead of the banks!