Sorry to hear of your troubles. Give Marriott some time to straighten it out and do the right thing. Since your stay isn't until January you do have some time. Might also want to PM the lurkers so they are aware of your issue. If they don't come through, then it is probably appropriate to take it up with your credit card company (e.g. dispute it as basically a "duplicate charge" and succinctly explain what happened on the MR call). Hopefully it won't come to that and at a minimum Marriott will credit back the charge for the reservation the call center just made for you. E.g. you wind up paying the original advance purchase rate you booked the room at.

--Jon

P.S. For the record, I am not a fan of disputing credit card charges as a primary way of resolving disputes with a vendor. In fact I have only done it for fraud or very, very rarely for a duplicate charge (only had that happen once or twice over the years). E.g, you should always work with the vendor until there is no other realistic recourse before disputing a charge. My two cents... YMMV....