********** transaction on Cheque Account

On the 20th of September @ 17h41, my cheque card was swiped for a fuel to the amount of R300.12 at Engen Cliffondale. The machine indicated that the 1st transaction was declined, I still have the slip. My card was swiped again, and this time the machine indicated that the transaction was successful. BUT the amount was deducted TWICE from my account, without the automatic reversal happening like it should for the 1st transaction. I already informed FNB, but it has been almost a week now WITHOUT ANY response. I also have statements from the Engen Garage that shows that the amount was banked only once on their side. On the form I submitted to FNB, it is stated that FNB will respond in 2 working days - WHATEVER?!! Can someone please tell me what on earth is going on here!? How can money just disappear into thin air like that!