MTN deducts money already paid to them

I phoned MTN retentions department to cancel a data contract. The cancellation fee I was told was R882.16 (comprising one months Subscription R99 and outstanding balance of R783.16). I made payment on 18th March by EFT in order for cancellation to take place. On 31st March MTN deducted again an amount of R783.16 from my bank account. When I contacted rententions I was told that R783.16 was an outstanding amount. I find this hard to believe as the amount is exactly the same as the amount I was quote for the cancellation, which I paid. So up to now MTN has received a total of R1665.32. I believe MTN has made a mistake. Further more when phoning them on *** I have waited a total of an hour before I have received service. I'm very disappointed and will discourage anyone from using this service provider.