Double charged now FNB sending me from pillar to post to reverse the second transaction

Am very disturbed that I have spent my whole morning trying to sort out a double debit of R1946. 41 for a transaction I did on my cheque account on the 23rd of December and to this moment this still has not been automatically reversed by FNB instead am being asked questions like I am trying to rob the bank of something so obvious. I have banked with FNB for nearly 20 years and have been double debited for one transaction, the least you could do is thoroughly investigate this issue instead of telling me I will be charged for proof from the merchant if that is needed as the proof I need is in my bank balance and the sms's I have forwarded to you and I told your consultant the merchant has confirmed that the transaction did not go through on their side. To say I am disappointed will be an understatement as I cannot wait for 14 days to have the amount reversed back into my account. I have never kept FNB's money for 14 days without interest so please give me the decent service I deserve in the way I have always conducted my bank account. Am very close to move to a different bank as there's nothing outstanding FNB does for me anyway.