double charged

I upgraded in october under the impression that the 'old' contract would be cancelled. When I received the bill the next month, it was almost double the amount of what the contract should be. I went to cradlestone cell c where the lady said to wait three months before I can change my contract. Three months has passed and when I went in to keywest today the lady informed me that the double billing consists of my old contract as well as the new one. I have been paying double the last 3 months for data etc that I am not receiving. In the past, when such a mistake has been made by cell c they would refund me. Today the lady said she can unfortunately not refund me. Then cell c wonders why their clients are leaving - I will be leaving next