Double billed on goods by Cashier

I used my credit card to pay at Clicks and it was declined. I then gave my cheque card which went through. Both transactions were deducted from both accounts with me paying double for the same thing. I queried it with the bank, FNB to reverse the transaction however Clicks will not give me the required Cancellation letter needed in order to reverse the transaction despite me providing them with proof from FNB that both transactions went through and was deducted off my accounts.