Vodacom double debts for upgrading

I have been with vodacom for more than six year and every time I have upgraded my handset I have never been debited twice for two hand set. Now vodacom is billing me twice because I have two months on previous contract. I was told by the sale consultant I will only be paying for the new upgrade and the old contract falls away because I have upgraded. Vodacom sales agents will do anything to make a sale and lie to the people. I am disappointed with vodacom. I demand answers vodacom or else I'm taking this matter further.