My limited knowledge of computer programming is that things like this are all or nothing.
Well it doesn't HAVE to be but really programming a computer to do something random takes ALOT more work then having it always do it the same way. Computers are bad at random

I would definitely think that a computer program to automatically cancel double bookings would check the number of diners, however it is slightly more complicated not because the ifs are hard (any 1st year CS student could do those) but just because Disney knows how many people your reservation is for and the dining program has your reservation number doesn't mean that the reservation system is sharing the number of people with the dining system. That would be the hard part to fix if its not already set up that way.
Now I have to go back and check all my October reservations. I have 3 at at time in some cases due to a party of 9 spliting into 3 groups. All but 2 are in my name since it was easier with me booking them all.