Changing interfaces on unmanaged hosts leads to an invalid host
If an unmanaged host has any interface mac address changed, either because it was re-provisioned externally, or some other external condition occurred, facts being uploaded with the new mac address in will result in a duplicate interface being created.
For example if a host has an interface 'eth0' with mac address 'foo' and is currently uploading facts to foreman but its 'eth0' network device changes mac address to 'bar', a second 'eth0' interface will be created.
If this occurs the host is left in an invalid state and requires the old network interface to be removed (in fact even trying to correct the situation via the UI leads to issues as it doesn't seem to be possible to delete the old primary interface in this case. At least not very easily).