Declining to refund

On the 22 of December 2017 I booked two vehicles online for rental using on credit card and insurance money was debited immediately for the two vehicles upon pick up was made aware that one credit card cannot book two vehicles and had to cancel the other car. As insurance was already paid was advised to contact the third party to claim the refund. I lodges lodged a claim for the unused vehicle and Nicole is declining with my refund claiming that I produced an invalid credit card. My credit card was valid and could cover for both cars but two cars couldn't be booked utilizing one credit card hence we cancelled. Nothing was communicated online while debiting two transactions with one credit card. Now I being sent useless and irritating email communication declining to refund me.