From the Alamo Insiders T&C's:
Discount not available with any other Contract ID and can not be combined with any other discount, coupon or promotional rate. Discount applies to base rate only.
The 10% discount is from the Alamo Insiders contract ID (7015135) that also provides a free additional driver. The contract ID you mentioned applies a 20% discount and replaces the 10% discount.
Thanks, I also found this on Mousesavers...
Additional Drivers
With Alamo, all additional drivers will pay a daily fee of approximately $10 per day unless they are traveling with relatives who are members of Alamo Insiders or have a Corporate Contract Agreement.
Alamo Insiders members in the United States are allowed to have the following additional drivers without a fee:
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Member's spouse, common law spouse, or domestic partner who has the same address on the driver's license as the member and who meet normal renter requirements.
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Immediate family member, Mother/Father, Son/Daughter, or Brother/Sister who has the same address on the driver's license as the member and who meet normal renter requirements.
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You also get a 10% discount on your rental (which can't be combined with other discounts or coupons). This discount comes from the pre-filled Insiders ID#. You can change this to another ID# if that gives you a better discount.
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Alamo Insiders members don't have to pay an additional driver fee in most cases.
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Alamo Insiders allows you to preregister all of your credit card and drivers license info, which saves you time at check-in.
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Alamo Insiders members can use the self-service kiosks, which is usually much faster than waiting in line at the counter.
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At Orlando airport, Alamo Insiders allows you go to straight out to the parking garage and check in at the booth there. There is usually no line at the garage booth. Thanks to Julie P for additional info.