Standard Bank

I made a transaction today on my debit card which "could not be processed due to a technical error" ( message displayed when I could not complete the transaction). I attempted th transaction again on my credit card and it went through. Also an sms from the debit indicating that transaction has gone through as well. Basically I paid for the same thing twice.

Called Customer Care only to be advised that I must wait for 2 business days for them to dispute the amount alternalitevely contact the merchant to advise them.

How is it the merchants fault / my fault for this because the 2nd payment would not have been done has I not received an error message?