account double debited

My account was double debited, I did a transaction on the 27/05/15 the same amount was again taken out on the 31/05/15. It took me an hour to explain this, even though the gentleman had my bank statement with him. After a lengthy debate and me telling him the place of purchase had proven to me they were in the clear. He still continue to say it wasn't the bank in the error. It's been 10 days... I'm yet waiting for a reponse... I feel let down by him and FNB. I love my bank. I feel they have changed the way banking is done....I wanna continue using them. Although my trust level has gone down for then dramatically..