But it is not an impossibility. Therefore there is some probability, however small, that it could happen.
As the number of clients increases, so do the odds of this happening. See the
Birthday problem article for more info.
Did you look at the
Probability Table on that page?
Even if everybody in the world was running a client you'd still not be getting close to a 10
−18 probability of a collision.
Admittedly this number is not zero but I never claimed it was impossible either.