Double debited from my account

I took out a personal loan with Sanlam in January 2025, They have been debiting my account every month end. Fortunately for me I got a permanent position at work and my pay dates changed from Month end to the 15th of every month, I called them in April to change the debit date. They said I was too late and the debit order will run at the end of April. They advised me to call on the first week of May to change my debit order to the 15th of June. I called them and arranged this, Yet this month the double debited from my account. I tried calling them and they cannot even assist me. How can you do this to your customers.