Debiting customers twice for the same month

My account was debited twice for December 2015 for the contracts I've got with Vodacom. They in turn said that the money will be credited back into my account. Instead they've decided not to credit my account from which the money was debited, but to take it as payment for the month of January 2016. Now I did not agree to this nor was I informed that this will be the case. My right to make my own decisions in this regard has completely been ignored and thus painting the picture that the right of the customer does not count as long as revenue is generated.