Cancel a payment

This morning around 2:15 am EST I was paying the bill for my father, who is currenlt in a Nursing Home Rehab due to illness. I tried to sumbit the payment and got an error message about an internal error and the payment could not be process, clicked go back and on the bill page there were not one but TWO payments listed!! One payment was for over $400.00! I did NOT authroize this payment. I checked all over about how to cancel and all I see is that I need to call my bank.  I called my bank and they said I need to contact Comast well. I tried to chat and I got nothing. I am really upset, you took MY momey!! My bank said if you cancel this before its paid 12/23, then there will be no issues. So who do I contact to cancel this while its still "pending" at my bank.