Unresolved double debit order.

Fnb connect debited R309 from my account on the 02/12/2020 for the month of December, on the 15th of December again they debited my account with R309 for same month, i called them and reported the double debit in my account but they failed to assist me. Can someone please help me with this situation because many people are loosing their money to the banks without knowing it.