Fnb debit twice with different date

Fnb debited on my account twice with different date, I have a personal loan with fnb and I asked them to debit on the 15th and they did debit on the 15th , to my suprised on the 24th today they debited again ,I called them and I asked them to rectify it and debit on the right date of our agreement which is the 15th and I was told that their unable to change it . All I want is fnb to debit on the 15th not the 24th