Double billing

I recently changed jobs and my salary date changed from the 1st to the 15th. I went to Cell C in August and had my debit order changed accordingly. The guy told me that the debit order would still go off as usual, on the first of September, but that it would change to the 17th in October. When the debit order went off on the 1st of October, I guessed the guy must have forgotten to change it and decided I would go back and ask them to change it when I got a chance, but then they deducted the full amount again on the 17th of the same month. I went in to the local branch who phoned their finance department and then gave me some story about how I pay 30 days later from the time that I am billed and that they are going to bill me again in November. I still can't figure out why I'm paying twice in one month, and even if this makes sense according to their books, why did no one warn me that I would be billed double? I work according to a tight budget, and an extra R500 a month on my cell phone contract is not funny. Won't stay on with Cell C past my current contract. Stick to Vodacom if you don't want surprises like these.