continous double billing

every month since February, MTN has billed me twice for hardware , when i call to complain i get an apology and a promise to correct it and give me a credit note for the error billing, they keep the promise but not untill i ,m charged an interest on the error billing, they also promise to rectify it, but the same thing happens the next month, when i call again, they do the same thing , rectify it , but it appears the next billing cycle again. i have spoken to them every month since February.