MTN taking double money refusing REFUND

I made an early fnb payment of R2102 via TRF 24/08 from my FNB Account and 27/08 MTN took R2052.09 again, double payment for one month now they refuse to refund me back my money. They say I was behind on my account but I get invoiced the 17 of the month then I pay on or before 25th of the next month, as it is states on the MTN APP when next payment is due and they always show on the MTN App my account is up to date, so looks like the app and Customer service at MTN doesn't correspond or not working. It's double payment and on FNB it shows it went through, not pending. I need my money back as soon as possible. I can afford to pay twice in on month, life is hard.