Unsatisfied Customer

I have been a DirectTV customer for the past 3 years and when my account was established, 2 accounts were opened with duplicative service.  For the past 3 years, I've been double billed and didn't catch it because I receive separate bills for internet and cable and have been paying my father's DirectTV bill, until he recently passed away.  I contacted DirectTV's customer support who validated that I have been enrolled and paying for duplicative service for years, but they advised that they could only reimburse me for 6 month's of the services that I have overpaid.  I don't understand why DirectTV cannot reimburse me the full amount that I overpaid for services and I am strongly considering cancelling my service.  I'm not sure how this error occurred, but they can clearly see that the accounts were for the same household.