Vodacom wont pay back incorrect Debit

My contract came to an end and i called to notify them, in advance, that i would not be upgrading and would like to discontinue. Firstly, the They debited me more than double what i was paying before the contract ended. They acknowledged the mistake, but I have been trying to claim the amount. It's been almost 2 months later, the amount is still not in my account despite the mails and calls and the invoice.