mon: Monitor: tolerate GV duplicates during conversion

Fixes: #4974

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge pull request #278 from ceph/wip-4974

Reviewed-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>