CenturyLink Billed me twice for one internet line

In Feb 2018 upgraded my internet speed with CenturyLink, and gave them a new bank account for auto-pay.  They did, but continued to take $ from my original bank account -- thus they double billed me for a single internet line.  Spent hours on the phone with customer service over several months, they often just terminated the conversation.  Twice customer service said they would fix it, but within a short time they were back to the double billing. 

Finally I had my bank reject any request from CenturyLinke on the original account.  That got their attention and I started dealing online to customeradvocacy@centurylink.com.  They said they would only refund half of the double billing, but it has been months and I have never received a refund.  First it was going to be a "credit" to my account, then they said they would send a check.  Latest is sort of "we do not understand what went wrong" and now they want me to start the whole process again with "creditdue.escalations@centurylink.com"  I do not know what to do now.  Are they just running out the clock and planning on me just giving up?