In a multi-master deployment, when a master starts up, it contacts the other masters to gather UUIDs, but doesn't check whether there are any duplicates. This can lead to some scary-looking issues, like duplicate masters showing up in the web UI. It would be nice if this were disallowed.
Also worth noting: having all unique entries in `master_addresses` doesn't necessarily imply they are all unique masters (e.g. some masters may be specified via DNS aliases).