American Express Amex double-charging for premium car rental protection Nationwide

American Express offers--to those who enroll--premium car rental protection which covers theft/damage for a "...flat rate of $19.95 / $24.95 per rental period, not per day."  However, I was charged $24.95 when I booked my car rental reservation and another $24.95 when I returned my car rental, even though they were for the same car/period!  I called Amex who ultimately reversed the double charge (after a lot of explaining), but I'm a bit surprised to learn that the burdon is on the consumer to make sure they have not been double-billed; Amex "washes their hands" of the double-billing by explaining that there's a clause in the policy which offers to refund double charges if you call them.  It feels like they must be making huge money in double charges when a system could easily be put in place which prevents double-charging for the same car rental.