How Do I Dispute Charge To Credit Card For Double Billing On Stretch Seating?

On return leg of trip, my wife and I upgraded to stretch seating during online check in. Our credit card statement was returned showing billing for two people to upgrade on first leg of flight. That billing was fine. It also showed a double bill for two seats on the return flight. In other words, where we should have been billed for two upgrades for two seats we were charged four upgrades (at $25 each) for only upgrading our two seats. Our credit card company told us to contact Frontier Airlines first.