Contract cancelled but still being debited

I cancelled my data contract with Cell C in November 2017. I was asked to send the settlement amount for the device which I did. However in December I was again billed for the contract. Upon phoning the retention department I was told that the contract was not cancelled and it seemed as if the money I paid was allocated to a different contract. I requested an immediate cancellation of the contract. However I was now again billed for January and the funds taken off my account. This is an ******* action from Cell C, who now have ****** my money for two months.