ct_hammer: submit dup [pre-]certs sometimes
ct_hammer: submit dup [pre-]certs sometimes

This is in addition to the parallel submission that is already
controlled by the MaxParallelChains option.

RJPercival/certificate-transparency-go,RJPercival/certificate-transparency-go,google/certificate-transparency-go,daviddrysdale/certificate-transparency-go,daviddrysdale/certificate-transparency-go,google/certificate-transparency-go,daviddrysdale/certificate-transparency-go,google/certificate-transparency-go,RJPercival/certificate-transparency-go