Request a refund on double debit

Vodacom has double debited my account after I had requested that they chance the debit date from the 1st to the 7nth of the month. All prior months have been paid. Vodacom refuses to refund this money but tell me that from now on i will pay for the services first. Looking at my current balance, it is still the normal should it not be at zero since I have already paid for February? All I need is a viable explanation as to why I cannot get ny refund