Double billed, no assistance from billing department.

I was double billed after doing an upgrade. Charged 2x device subscription fees in one month.

Case lodged on 10 October 2018. The manager on duty agreed that there was an error and that I have the right to log a dispute. No response yet. I cannot be assisted in store, and have received no assistance from the call centre.

What am I supposed to do? If I stop paying I will be blacklisted. Surely this is *********?