Poor service and no refund on double debit

I renewed my main cellphone contract with Mtn in May 2021 at Wonderboom Junction in Pretoria North. I had to pay an amount to close off the old contract as it was still active(I forgot what's they call that balance). I had the option of paying via debit order(next debit order) or pay cash in store. I paid in the store by debit card. On my next debit order Mtn debited that amount again together with my new monthly subscription. Another issue I had was that Mtn altered my contract date for my other number(data Sim) . I signed in January and they deducted from my account with immediate effect yet on their system, instead of Jan 2021 they put May 2021 which means I will pay for more that 24months. I contacted mtn and they asked me to go to the store to inform them. I went to Mtn Wonderboom Junction to query. I provided the slip and the contract I signed in Jan 2021. They agreed and informed me that they will rectify it. I have been calling. A lady told me to come to the store again because they are overwhelmed and attend to walk-ins more. How many times must I go there? The person who assisted me with the renewal keeps promising to rectify and get back to me but nothing is happening. I am still not getting my credit.