While upgrading a MediaWiki installation I ran into the error

Query: INSERT  INTO `actor` (actor_name) VALUES ('RedirectBot')
Function: MigrateActors::addActorsForRows
Error: 1062 Duplicate entry 'RedirectBot' for key 'actor_name' (localhost)

while running maintenance/upgrade.php. The problem bit quite a lot of people with older MediaWiki installations. And while the official bug report is mildly interesting to read, there hasn’t been a solution since July 2019.

In my case it was rather easy to identify the problem. MediaWiki saves the username and user_id with every revision. In this case the user RedirectBot had two different user_ids, 860 and 200012. I fixed this by making sure that all revision entries for RedirectBot have the same rev_user: