Incorrect Bill

This month I paid more than double for my contract with MTN. When I investigated this, I saw on my bill that they added someone else's contract to my account. I phoned the ***** Dept and they said it would take 21 working days for them to return my money after I've sent then an affidavit and certificied copy of my ID. This is totally unacceptable! I HAVE TO WAIT 21 DAYS FOR A MISTAKE MTN MADE!!!