Double debit

I had a double debit of R3176 and Im left with nothing for December, they were suppose to debit me on the 27th of Nov and they didn't and when they called me informing me they didn't debit and I l found out they had my debit Date wrong and the Gent I was talking to told me they will debit in Dec and I asked if I won't be debited twice he said I won't they will note Dec as my 1st debit month but I was debited twice R6300 in December and when i told the manager of the branch she ignored me. I don't think this is legal!