Is that check really necessary?
To have 1 in a trillion chance of having accidental SHA-1 collision they'd have to store 1.7*10^18 keys, and mere key index of that would require 54000 petabytes of RAM.
Accidental sha-1 collision is probably not a problem, but in a few years [1] it will be possible to crate sha-1 collisions and use that as an attack. It looks difficult, but supposes that with the correct string an attacker can retrieve the cached information of another user, for example sha1("joedoe:creditcard")=sha1("atacker:hc!?!=u?ee&f%g#jo").
I don't know if they are using randomization, because the collision can be used (in a few years) as a DOS atack [2]
[1] http://www.schneier.com/blog/archives/2012/10/when_will_we_s...
For instance MD5 collisions are really easy to create but for preimage attacks on MD5 there is still no better approach than just doing brute force.
By then, maybe the founders of this system won't even be alive.