If a replica set somehow gets into a state where
there are two primaries, and the two primaries both see a majority of
the replica set but not each other, then neither steps down.

This happens because only the two primaries try to resolve the
situation (see Manager::noteARemoteIsPrimary). In the test, because
the primaries don't see each other, they both mistakenly think they
are the sole primary, and do nothing.

If some other machine in the replica set sees two other machines are a
primary, that machine does nothing to try to resolve the situation. It
does not send a step down command to either primary. See
Manager::msgCheckNewState().

See: https://groups.google.com/forum/?hl=en-US&fromgroups=#!topic/mongodb-dev/WA--aofOjQI