Verified in Satellite 6.3.1 Snap 2.
I ran content-host-d's flood script to register 5000 new content hosts within 1h 25m. During the process, there were always 50 concurrent registrations. At no point did any of the hosts encounter the "host_id has already been taken" error.
The only two registrations that failed were due to this unrelated error:
"Unable to register system, not all services available"
Which is likely due to the Satellite struggling under the registration load.
The failed systems were bad1835 and bad3606.
I am attaching the logs for the script as well as all of the registered clients.
# python flood.py -s ibm-x3550m3-10.lab.eng.brq.redhat.com -t rhel72 -c 5000 -n bad --limit 50 --exit-criteria regis
project source: https://github.com/JacobCallahan/content-host-d
Created attachment 1417261 [details]
Created attachment 1417262 [details]
this was released in 6.3.1.