Incorrectly Charged

I was incorrectly charged for internet at an address I’ve never lived at. I thought my address was corrected and ended up discontinuing services and returning my equipment (both of which i have proof for). Turns out xfinity just made another account with my correct address. The bill for the address I’ve never lived at is now in collections and I’m being told I owe it even though I’ve never lived there and haven’t even used xfinity in half a year. I spoke with an agent and they said I don’t owe anything and would dispute this with the collections agency but I got text saying it was unable o be completed. I don’t know what do.