Double billing of data when on a topup contract

My contract moved over from Nashua Mobile to MTN last year November 2015. The billing was R299.00 a monthly subscription of R200 on a Topup contract and 500MB of R99.00 recurring hybrid bundle. On the 01.03.2015 by bill escalated to R887 and the exact same amount on the 01.04.2014. Double Billing of bundles charged to a value of R570.00 extra a month. When queried at the branch the consultant had no idea why this was done. I was told that a USD was logged on the 08.04.2015 and that the query would take 7 to 12 days to be sorted. After numerous calls no feedback has been given to date. Tomorrow is the 01.05.2015 and new billing will be done again and my query has not been resolved. How can a topup contract incur extra costs this is ****************!!!! I was told I need to pay this amount or my sons cellphone account will be suspended. Quick to want to received payment but not quick to resolve network billing issues. NOT a HAPPY Customer be warned people and check your bills. What has happen to efficient service