Enforce that an option name isn't registered twice in a scope.
Review Request #3695 - Created April 15, 2016 and submitted
|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
CI passes: https://travis-ci.org/pantsbuild/pants/builds/123318109
Revision 2 (+34 -10)