While testing concurrent Katello registrations in batches we see the following error leading to some hosts (2, 3 usually) with the same OS version failing to register:

PG::UniqueViolation: ERROR: duplicate key value violates unique constraint "index_operatingsystems_on_title"
DETAIL: Key (title)=(RedHat 9.2) already exists.

After these errors, the rest of registrations work without problems.

The error occurs with 50 concurrent registrations but does not occur with >120 concurrent registrations.