Charged twice for the same transaction

On Sunday the 30 August 2015. I filled my car at Engen Lyttelton and paid by credit card. The first time the guy that assisted me said that it was declined and can he try again. I agreed as I knew there was funds available and this time it went through. This morning I see on my bank statements that the amount which was R577.50 was deducted twice. I want this immediately investigated and the extra money taken given back.