This is what someone explained to me, from a math perspective:

If 137 Billion people generated 137 Billion addresses per year for 137 Billion years, there would still be only a 1 in 35 Billion chance that ANY of them were the same.

Regardless if those numbers are true (someone smarter than me can tell us), for all intents and purposes, the odds are really, really, really, really, really, really, really, really small. Really.