Hopefully you're problem has already been solved. However, I'll still share my experience.

I was double charged for a Priceline hotel stay last year. The horrid thing was that I was charged more by Priceline than by the hotel (and yes, I'd done all the research before hand). Once I say the two charges, I immediately disputed both with my AMEX. I then called Priceline and they eventually fixed it. In the mean time AMEX put both charges in dispute.