Incorrect additional debit

My card statement has a "Payment due date", this month the 3 April. As usual and as the last 15 years or so, I made the payment required and my bank statement, at 06:30 when I checked, showed the debit dated 3 April. Additionally my online Discovery card statement, also checked at 06:30, showed the credit dated 3 April. WHY then was a further substantial amount also debited from my bank on 3 April? The meaning of "Payment due date" can be none other than the date provided, not a day before or anything else. As usual, I have had absolutely no response from the call centre. I am disgusted to know that Discovery can play fast and loose with my money and I wish to discus the ramifications of this gross error. Contact me please.