This question already has an answer here:
No SHA-1 collisions are actually known, though there are a number of cryptographic attacks to weaken it.
But, how likely is it that, of all the SHA-1 hashes computed since the invention of the algorithm, there has been a collision?
Not sure how to approach this, but it probably involves the Birthday Problem?
(if there's a better SE to ask this in, let me know)