Comcast Comcast insists on charging me for a non existing modem! Harford County Nationwide

 I'm 63 years old and have never worked with a company worse than Comcast. Comcast insists on charging me a monthly modem rental fee on a non existing modem. They say I have two modems? Both are the same model with a serial number off by one letter?? The truth is, on May 26, 2013 I purchased a modem through Amazon for $66.91. My wife called in the information so Comcast could set the modem up. The number was read wrong and then immediately corrected (contained D not O). The service rep, instead of correcting the number added another modem with the corrected number. Every couple of months Comcast tries to charge me for a non existing modem. It usually takes several calls and a couple of hours of my time to get it corrected. Online it still shows two modems on my account. Comcast won't take it off. Under the fictitious modem data usage always says " We're sorry. We can't load your usage meter right now." GEE I WONDER WHY!