Does anyone have experience with having duplicate reservations - same location, same (or close) date/time, different car class?

I am thinking about doing this as a way to avoid getting a Town Car when I reserve Group H. If a TC is at my space with no unassigned DTSes on the lot, use another reservation for a Group E. I refuse to pay Group H prices for a Town Car, a much inferior product to the DTS.

The Group E reservation should theoretically be upgraded to Group G as an Avis First member. Of course, Group G seems to be infested with Crown Vics and Grand Marquis rather than the Lucernes and 300s I prefer.

Since both of these would be preferred reservations, I am wondering if the computer would catch this, as an airline's would, or if one of the credit card authorizations would be denied.