I misread "check that this pair has not yet been processed," and agree that pre-processing all pairs is not a good approach- I had thought you could remove individual pairs.

But if he did take that approach, he would be able to say "my script will finish," which you can't do if you retain known invalid pairs. It could sit there pulling the same candidates, if it is truly random... but then, Eris needs her toys.

And just because one of my friends recently said he'd never seen it posted on a forum, I feel obliged to throw this out here too: I was wrong. :)