You guys forgot about so called Birthday attack. No, I am not talking about USA drones bombing wedding parties or funerals in Afganistan and Yemen. I am talking about this one
https://en.wikipedia.org/wiki/Birthday_attack when lots of random numbers are generated, it greatly increases probability that two random numbers are identical. Somebody should do the math to calculate exact percentage on how likely is to get one collision when there are n numbers of unique bitcoin addresses generated and used in blockchain. But I still think that probability of collision in real life is close to zero, even if birthday attack collision probability is increased several orders of magnitude.
The general Birthday Problem answer applies: For 160 bit addresses, a collision becomes more likely than not when approximately 2^80 addresses have been generated.
How big is 2^80? Presently there are around 2^33 humans. The human lifespan is around 2^31. If every human generated one address every second their entire life, that would be 2^64. There would need to be 65536 earths before the collision chance crosses 50%.