How does the bitcoin network disallow people generating a key pair already generated? Is it possible to generate a key pair that has already been generated? Wouldn't it collide with an existing value? 

I assume it is mathematically very unlikely, but odds higher than 0 are still possible.
Yes, it is possible to create an already generated and in use address. The chances of that is indeed larger then 0, but minimalistic. But really small. And if I say really small, I mean extremely small. And if I say extremely small, I mean very extremely ultra low super duper close to zero. It is definitely NOT something to be considered as a weakness, never ever (because it will cost more energy before you have 50% to hit a duplicate address then the whole sun provides in its whole life time then). source