In all likelihood, the "robot" will kill one of your duplicate reservations. This software routine runs every 24 hours and, among other things, is specifically looking for this sort of circumstance. IIRC, it will be the most recent reservation made that will be killed (which seems strange, given that the most recent reservation made would seem to represent a person's most current wishes).

A couple of years ago I had a discounted award ticket (miles required were reduced by 10 or 15 thousand as part of promo) to OGG in place but then a potential business trip to Asia came up. The business trip wasn't a "for sure" thing but I wanted to get a reservation and upgrade requests in place ASAP just in case. Bought a full Y fare but it was killed within hours. No amount of pleading or calling back rectified the situation. I was told that the Robot "realized" I would be airborne on two separate flights simultaneously and therefore killed the most recent reservation automatically. That's a slightly different circumstance but I'd be willing to wager that essentially the same thing will happen if you try to double-book the same flight in the way you describe (no doubt there are manual overrides possible for large people wishing to purchase two seats, etc). And the risk there would be that the Robot kill your original reservation and you lose your cleared upgrades!