Wouldn't the agent know pretty quickly that you were trying to pull a fast one? I can see them getting annoyed with the 2 reservations once it dawns on them that you did it intentionally - especially at an HLE that only has a few cars to begin with. Holding the extra reservation might have meant they can't rent that car to someone else.

Besides, whenever I rent at an HLE I have a hard time getting them to not upgrade me - they usually have a surplus of bigger cars, including the V8 SUV's, sitting around. (Usually when I'm booking HLE it's because I want to put 1,000 miles or so on a car in a weekend, and therefore don't want anything bigger than a 4-cylinder Toyota or Mazda.)