Enforce that an option name isn't registered twice in a scope.

Review Request #3695 — Created April 15, 2016 and submitted — Latest diff uploaded

nhoward_tw, stuhood, wisechengyi

Amazingly, we didn't check for this before, at least not since we
got rid of our use of argparse.

This commit fixes the one case where we actually did double-register.
The fix required a bit of a hack in export.py, but there's a TODO for that,
and we need to revisit export.py anyway
(see https://github.com/pantsbuild/pants/issues/3200).

CI passes: https://travis-ci.org/pantsbuild/pants/builds/123318109