Sent to collections despite closing accounts

I am repeatedly receiving messages from Waypoint Resource Group stating I owe debt related to a Comcast account.

I at one point had TWO active addresses with xfinity. I closed both out and returned my equipment for both addresses. Despite closing them out and being assured of this, it took months to get xfinity to stop auto billing me due to one of the accounts being connected to an xfinity mobile plan. 

how do I ensure this collections report doesn’t impact my credit?? I was assured each time I called that my account was finally fully closed out.