It’s quite a delicate balance, so I’m not even sure this change is “correct”
main ← fix-duplicate-topic-titles-visibility
merged 01:35PM - 24 Jul 26 UTC
Previously, the duplicate topic title check compared new titles against every to… pic on the site regardless of visibility: users were blocked by titles in categories they couldn't see or unlisted topics they couldn't find, the bare "Title has already been used" error gave no way to locate the conflict (and doubled as an existence oracle for hidden titles), and the behavior was controlled by two entangled boolean settings.
This change scopes the check to the destination category plus whatever the acting user can actually see, links the conflicting topic in the error — safe by construction, since being blocked now implies being able to see it:
> This title has already been used by [another topic]().
It also consolidates the two booleans into a single `duplicate_topic_titles` enum (`disallowed` / `allowed_across_categories` / `allowed`), with existing values migrated and the old names kept as hidden deprecated aliases that admin search still resolves.
Reported in https://meta.discourse.org/t/title-has-already-been-used-in-a-secure-category/123047
Note for self-hosters: env-provided settings can't be migrated — `DISCOURSE_ALLOW_DUPLICATE_TOPIC_TITLES=true` configs need to switch to `DISCOURSE_DUPLICATE_TOPIC_TITLES=allowed`.
The idea is that we’ll only ever block if you can see the other topic. If you can, we’ll even provide a link to the topic so it’s a one-click to view it.
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