Comcast not making good on overbilling!

Over 2 years ago I switched one of our 2 cable boxes from HD to DVR. It took 5 tries before Comcast had it all up and running correctly. Now I discover that they never credited us for the return of the HD box. We have been charged for that for over 2 years. I know we should have picked up on it but the way my husband read the bill he thought we were just paying for the 2 boxes not an extra 3rd box. Their records show that we are only receiving service on the DVR and one HD box but they have billed us for 2 HD boxes and a DVR box. They will only give us credit for 6 months of their mistake. This doesn't seem right to me. If the reverse happened and they under billed us for 2 years and discovered the mistake I am sure they would come after us for the entire amount. I may hand it over to an attorney. Comcast makes me furious!