how about some double check?
I'm no guru when it comes to this, but in my mind it does not seem worth it to double check sieving. It would cost alot of CPU time and save very, very, very little CPU time.
In a way, you could say that the PRP tests are double checking the sieving effort anyways.
And from what I understand, all reported factors are validated upon reporting, so it will not happen that a kn-pair is falsely excluded. A double check sieve would then only be useful for finding factors that was lost the first time around, and I doubt there are many.