Transferred Service and Have Been Getting Double Charged. Can't get Xfinity to help with the refund. About to cancel service completely

I moved at the beginning of July. At the end of June, I initiated a transfer of service to my new address. This was completed on July 3rd. From August-November, I was charged for service at my old address and my new address every month. The charges on my old address added up to ~$600. After speaking with customer service several times, I finally went into an Xfinity store and they were able to stop the charges from continuing. They said I would be receiving a refund soon and to come back if not. Today I went back in as instructed, they submitted a refund request ticket, and within 20 minutes I received a text from Xfinity saying the request was denied and the ticket was closed. I called customer service only to have the call drop after an hour and half conversation with the representative. 

I have been a customer for 10 years. At this point, I don't know what else to do besides pursue legal action and cancel service completely. Has anyone been able to get this sort of thing resolved? Xfinity has been completely incompetent in the matter, and I have all the proof/paper trail I need to successfully resolve this with legal action if needed.