If I were ever double billed -- and I think it can happen by accident, especially at a new restaurant where they're still learning how to use the machine -- I wouldn't even bother calling the restaurant. I'd call my creditcard issuer, who would take care of it right away. It's just a whole lot easier dealing with the credit card companies than with the merchants.

BTW, I once was significantly overcharged by a NYC restaurant, although I'm pretty sure that it was accidental on their part (missing the decimal before the cents). I got a call from Citibank that very same day, before I had even discovered the error -- I guess they spotted the amount charged as being unusual for this particular restaurant -- and the problem was rectified in a 30-second phonecall.