[users at bb.net] New masters being created after restart

[Lukas Jirkovsky]

Hello,
I have a problem with new masters being created after every restart.

We are running buildbot-master using docker image
buildbot/buildbot-master:v2.7.0. The database is postgres 12.2 also
running in docker. Whenever the container is restarted, a new build
master is created. This has happened quite a few times recently for
technical reasons when the server hosting buildbot had to be
restarted.

The list of build masters now shows a lot of masters with all except
one being inactive. I think this has also caused an issue where two of
the builders no longer work despite their workers are online, because
I can see that the workers are now listed with two build masters which
has never happened before.

Attached is the image of how the list of build masters look.

Any ideas how to:
1) fix the issue so that it doesn't occur again? I didn't find any
related bug in the bugtracker and I'm not sure it isn't caused by a
wrong configuration.
2) cleanup the mess from the older restarts?
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