Poor service from MTN

Mtn offered a contract during black Friday week. I had an contract which was due for upgrade I contacted mtn to upgrade and was accepted on 27 November 2023. I received a invoice from mtn debiting me for the period 27 November 2023 to 26 December 2023 on my old contract I have received a second invoice for the same period on my upgraded contract. They are not prepared to reverse the duplicated debit. How can MTN be allowed to charge clients twice for the same period