Incorrect Bill (Double charge)

I took out a contract at Vodacom for an s8 plus. They supposed to charge me R799 a month and give me 20 gigs once of data. In the middle of the month I noticed my bill was around R1300. I gave them a call and they advised the excess amount will be reversed as it is just for Data that is free. Today they charged the full amount. I would like this amount reversed. I'm shocked at a company the size of Vodacom does not have their accounting department in order.