MTN Insurance Double billing

I have an on going query with Mtn insurance, they have been billing me twice for one device since January. I called the whole of April to date they are still double billing me. every time I speak to their agents they tell me that they only see one insurance charge but when I send them my invoice they can see what I'm talking about. I am very unhappy with their services. I have sent countless emails to their supervisor by the name of Charmaine and until today she has never responded, even when the agents say that they will ask her to provide feedback. it is so easy for them to debit my account but very difficult for them to credit me back. I need this to be solved because I will not continue to pay a double charge for one device.