Sixt Charged My Credit Card Twice for the Same Car Rental

I booked a 6-day car rental through Booking.com from Sixt at Mulhouse-Basel EuroAirport for our Switzerland trip from March 13–19, 2026, making the full payment including a complete protection package on February 24, 2026. When we arrived to pick up the car on March 13th, we were told we needed to purchase the full protection package directly from Sixt as well, so we bought it again through them. They also recommended we purchase winter tires, which we did. Since our reserved vehicle was unavailable, they gave us a higher-tier model instead. We returned the car at the end of the trip without any issues.
After returning home, we requested a refund for the original full protection package we had purchased through Booking.com — and to their credit, Booking.com handled this promptly and refunded us right away, so thank you for that. However, on March 23, 2026, Sixt charged my credit card a second time for the car rental fee. I have no idea why this happened.
I am demanding that this situation be investigated and that the duplicate charge be refunded to me in full.