double billed for a year because of base station upgrade and failure to close out old account

so I am looking at my bill and notice that  I have been charged double for the past year. I spoke with representative and was told that when I recieved an upgraded system, the old system should have been cancelled. Company will only credit me for five months and not the full year of overcharging. 

I didn't ask for a new system. It was sent out because the old system was obsolete. I don't recall being told to cancel my old system. Further, my dashboard reflects two variations of the same address. This practice is unethical and unfair.