hohan,
Yes, it is a double discount but it's a "Contract Number" and then a "Coupon Number" that you give.
For example, I belong to the Disney Club and they are giving one of the best discounts with the "contract" number (20%) so I plug that in where it says to then I've got a $20 off coupon that I received in the mail from my credit card company. I can use them both.
Also, I believe, but I not sure, that you'd save more money doing your reservation online. Usually rates are better online.