Duplicate entry 'unknown' for key 'actor_name'

Wikimedia\Rdbms\DBQueryError from line 1699 of /var/www/html/includes/libs/rdbms/database/Database.php: Error 1062: Duplicate entry 'unknown' for key 'actor_name' (db) Function: MigrateActors::addActorsForRows Query: INSERT INTO actor (actor_name) VALUES ('unknown')

#0 /var/www/html/includes/libs/rdbms/database/Database.php(1683): Wikimedia\Rdbms\Database->getQueryException('Duplicate entry...', 1062, 'INSERT INTO `ac...', 'MigrateActors::...')
#1 /var/www/html/includes/libs/rdbms/database/Database.php(1658): Wikimedia\Rdbms\Database->getQueryExceptionAndLog('Duplicate entry...', 1062, 'INSERT INTO `ac...', 'MigrateActors::...')
#2 /var/www/html/includes/libs/rdbms/database/Database.php(1227): Wikimedia\Rdbms\Database->reportQueryError('Duplicate entry...', 1062, 'INSERT INTO `ac...', 'MigrateActors::...', false)
#3 /var/www/html/includes/libs/rdbms/database/Database.php(2343): Wikimedia\Rdbms\Database->query('INSERT INTO `ac...', 'MigrateActors::...', 128)
#4 /var/www/html/includes/libs/rdbms/database/Database.php(2323): Wikimedia\Rdbms\Database->doInsert('actor', Array, 'MigrateActors::...')
#5 /var/www/html/includes/libs/rdbms/database/DBConnRef.php(68): Wikimedia\Rdbms\Database->insert('actor', Array, 'MigrateActors::...')
#6 /var/www/html/includes/libs/rdbms/database/DBConnRef.php(369): Wikimedia\Rdbms\DBConnRef->__call('insert', Array)
#7 /var/www/html/maintenance/includes/MigrateActors.php(219): Wikimedia\Rdbms\DBConnRef->insert('actor', Array, 'MigrateActors::...')
#8 /var/www/html/maintenance/includes/MigrateActors.php(412): MigrateActors->addActorsForRows(Object(Wikimedia\Rdbms\MaintainableDBConnRef), 'rev_user_text', Array, Array, 1473)
#9 /var/www/html/maintenance/includes/MigrateActors.php(105): MigrateActors->migrateToTemp('revision', 'rev_id', Array, 'rev_user', 'rev_user_text', 'revactor_rev', 'revactor_actor')
#10 /var/www/html/maintenance/includes/LoggedUpdateMaintenance.php(45): MigrateActors->doDBUpdates()
#11 /var/www/html/includes/installer/DatabaseUpdater.php(1343): LoggedUpdateMaintenance->execute()
#12 /var/www/html/includes/installer/DatabaseUpdater.php(512): DatabaseUpdater->migrateActors()
#13 /var/www/html/includes/installer/DatabaseUpdater.php(475): DatabaseUpdater->runUpdates(Array, false)
#14 /var/www/html/maintenance/update.php(181): DatabaseUpdater->doUpdates(Array)
#15 /var/www/html/maintenance/doMaintenance.php(107): UpdateMediaWiki->execute()