Comcast Comcast billing rips its customer off Internet

I have been using Comcast for more than ten years. I think something has gone wrong with Comcast billing system, because I have received conflicting bills and I have been over charged. Sometime in June 2012 I called Comcast and told them I would be moving from PA to MN. The person who answered the phone told me I must disconnect my service in PA and then schedule a new service in MN. I told her to disconnect my service in PA on JULY 20, 2012. Unfortunately, my Comcast service was disconnected on JUNE 20, 2012. It took me a couple of days to clear that problem. Then, Comcast billed me on June 23 for $128.49, despite the fact that I already paid the bill in full ($68.55) on June 5th. After I paid another bill in full ($68.55) on July 6th, Comcast billed me again on July 13 and then on July 19 with a huge amount. When I talked to a Comcast agent (Sherry) to discontinue my service on July 20, she told me not to submit any payment after I paid the July bill. But Comcast charged me twice in July. When I talked to a supervisor "Leah," she said the information from "Sherry" was not correct. Without  realizing that my bank had already paid $68.55 on July 5th, I made another payment ($59.49) on July 23, 2012. When I received my bank report and found I paid twice in July, I called Comcast about the payment. It took me almost an hour to reach an alive person, because I was transferred from one agent to another and then got disconnected. Supervisor "Leah" refused to refund the second payment ($59.49) I made in July. 

After moving to St. Paul, MN, I had my Comcast service installed on July 23, 2012. A few days later, I received a bill from Comcast, asking me to pay $91.83, which included one month Cable TV/ Internet fee and an installation fee. The bill indicates the due day of payment is August 18, 2012. On August 13, 2012, Comcast issued another bill asking me to pay one month service in the amount of $48.97 plus the $91.83 from the previous bill. In other words, between July 23 and August 13, 2012, I was asked to pay two month Cable TV/ Internet fees, which does not make any sense at all. Again, Comcast rips its customers off. What can I do to get justice?