I was charged twice for the same i tunes purchase

Hi ,

Hope i can get some advise on what i should do . I was declined by itunes on my purchase and when I checked the credit card I saw that my security code was missing on my file so I added it and tried again and again it was declined . So I added a Visa credit card to this account and my purchase then went thru. Then I checked with my bank why my mastercard was declined and they said that it was not and that the purchase went thru the second time . i just receieved confirmation from both my credit cards that my purchase went thru so i have been charged twice ( once in my visa and once in my mastercard. for the same purchase . Can i get help on who I should contact with Apple on getting a refund . I tried to do this online but have not been sucessful. Thanks