Unacceptable service delivery

I visited the vodacom shop in the Goldfields mall on 18 Dec 2017 to enquire about my large vodacom account. A call was locked and I follwed up on 19 Des 2017. I phoned vodacom regarding my cell phone account which they deducted double for the handset for Nov 2017 and Dec 2017, although I updated already on 21 Sept 2017. In Oct 2017 the correct amount was deducted. They assured me that the account will be credited with the double deduction, but when I phoned them today, 15 Jan 2018 @ 14h35 they still did not resolve the problem and double charged me again for January 2018. Please help!