Multiple duplicate charges on FNB credit card

In the process of switching to a different credit card account, FNB duplicated several charges to the value of more than R6000. When I contacted them, they insisted that I had to fill out dispute forms to dispute each transaction (one form per duplicate charge) and that it would take 30 days to process, even though the issue wasn't caused by either myself or the merchants. The dispute form requires a reason for the dispute, and of course there is no option to select "FNB admin error". I also contacted the personal banker who dealt with the credit card migration, and they can no longer help me as they no longer have access to my profile. A really poor experience, after many years of being an FNB customer I expected to be treated better than this.