Submit duplicate certificates and precertificates

[jsha]

To catch conditions like this non-incorporation of SCTs for duplicate certificates: https://groups.google.com/a/chromium.org/forum/#!topic/ct-policy/E88pjOZzkIM

[cpu]

We talked about this in a call yesterday: it's simple to have the certSubmitter submit duplicates of its own precerts/certs. We should do that as a matter of diligence but outside of this issue. It would be more interesting to use this issue to track submitting copies of certificates from observed from calls to get-entries on the CT server.

[jsha]

Note: This can be done by choosing a random index smaller than the tree size, fetching it, then re-submitting it.

[cpu]

it's simple to have the certSubmitter submit duplicates of its own precerts/certs. We should do that as a matter of diligence but outside of this issue.

Tracking that in #54