Double Billing

I just found out that Tracker has been double billing me every month from October 2016 till to date. Instead of paying R107.85 each month, I have been paying R248.06. When i questioned them yesterday they could not give me an answer nor could they find fault on my side, they just admitted that they have been double billing me. When requesting a refund, they advised me that they will only be refunding me for 12 months, which i find very unprofessional and unacceptable as the fault was/is not mine. Their accounts Manager should have picked up the error a long time ago.