In test net, it looks like we see nodes that have re-registered and have a new identity, but have messages they generated under an old identity in their Artemis journals. I've also heard reports of people accidentally re-registering (e.g. by leaving command line args set on node start command - not sure if still possible).
We should prevent a node re-registering with the doorman. The only time they should be able to register is if they do not have any "state". Database/artemis/files should all be empty.