Double charges from Verizon. What should I do? Thanks.

About one year ago, I switched from Verizon DSL to Verizon FIOS. The DSL was supposed to be disconnected from then. Just recently, I found Verizon charged me for both FIOS (direct from credit card) and DSL (separate Verizon bill). I called Verizon and was told they can only refund DSL charge for three months. They also argued that I also had responsibility for checking my bill every month. What should I do? Thanks for your kind help.