Redmine

Did you use the Trac importer for this project ?

Ah, yes I did.

That's a bug in the Trac importer. I'll fix it to prevent this problem in the future.
Go to projects settings, uncheck the trackers, save, re-check them and re-save. You should no longer see duplicates.

I think that was enough of a hint. The database looks like this:

redmine=# select * from projects_trackers;
project_id | tracker_id
------------+------------
1 | 1
1 | 2
1 | 1
1 | 2
(4 rows)

Deleting the duplicate rows solved the problem. Perhaps this table should have a unique constraint across both columns? I believe this would remove the need for the existing index on project_id, as well.