FNB took my money, they must get it back

FNB credit card division took money from my account after I explicitly told them that the query was resolved. There was a double transaction on my account on the 28th of December. FNB reversed the 2nd transaction, but then the merchant also refunded us via EFT. My wife then refunded the merchant via EFT so everybody was happy. On the 20th of February 2018 someone from FNB credit card phoned me about the query for 28 Dec 2017. I told the person that it was settled, but he still proceeded with taking this money from my credit card with the merchant from 28 Dec as the beneficiary. I contacted the merchant a few times in that week and they confirmed they never received the money and I need to contact the bank. I submitted a complaint to FNB credit card division and they sent an email to me saying that I should get the money back from the merchant myself. I've been to a FNB branch and the lady was very helpful but she can't fix the problem, it needs to be resolved by the credit card division. I've submitted a credit card dispute form twice and just get no feedback from FNB. Tomorrow, it will be a month. This is totally unacceptable. FNB took my money, they must get it back