We are also experiencing this same issue for a long time now - of course there’s no way to reproduce how the database got in this state but every so often we end up with users either self-renaming to duplicate names during the rename grace period, but this also applies to admin renames.
We don’t use unicode usernames, and this is pretty.. problematic at times as this breaks the Patreon plugin’s sync job at times which we heavily rely on for external policy services.