Being Double Billed for an old Account and new account after speed upgrade

I need help for being Double Billed for an old Account and new account after speed upgrade  since June 2026. Xfinity messed up and ended up having 2 accounts on my address.
I called Xfinity customer service  and the agent looked it and said they fixed everything but I was still double billed since then. I contacted them again last week and once again they said they fixed it. But after that I got a separate bill for this old account of 2-3 month service charge plus equipment charge which I only used my own device. I've lost hope now with Xfinity customer service. Please help.