Nedbank has the worst service ever

On the 25th of August 2017, I made an online purchase using my card number. The orders were duplicated, therefore I was double charged, I immediately called the bank to notify them and I was told they could do nothing about it. I requested a refund with the merchant and they confirmed the funds were never taken, their system did an automatic reversal into my account. The bank insists on not having a record of that debit, and they refuse to refund me. According to the bank the dispute was invalid and I needed to contact the merchant, which I did, the merchant says I must query this with the bank. Had they attended to my query when I 1st call them, I wouldn't be sitting with an outstanding amount of R1900 from my account, which the bank won't account for :(