Nothing, just the fact that generating the same private key takes like what, more than the life of the universe?
Perhaps this is a better comparison. What's the worse thing that could happen if you generated an address that is already in use? What if you would die as a result?
Let's assume there are 1 trillion (1,000,000,000,000) addresses in use currently (it is most likely much less). The chance of generating a address that is already in use is then 1,460,000,000,000,000,000,000,000,000,000,000,000. The chances of any particular person in the U.S. dying in the next second are about 1:3,000,000. So,
an average person in the U.S. is 500,000,000,000,000,000,000,000,000,000 times more likely to die in the next second than to generate an address that is already in use.
I think you are pretty safe.
