FYI, I've hot-fixed this on plwiki through MediaWiki:Vector-2022.css. If reproducing, use safemode or disable this style in browser console.

It seems to be correlated with FlaggedRevs... – I see this bug on plwiki, dewiki, ruwiki, ruwiktionary, but not enwiki, eswiki and frwiki

A couple of recent changes to flagged revs this week: https://gerrit.wikimedia.org/r/q/project:mediawiki/extensions/FlaggedRevs+status:merged+owner:ihurbainpalatin@wikimedia.org

https://gerrit.wikimedia.org/r/c/mediawiki/extensions/FlaggedRevs/+/1244744/6/includes/backend/FlaggedRevs.php fixed a bug where flagged revs stored both the legacy and parsoid output in the same parser cache.

But this bug seems to affect both parsoid and legacy.

Confirmed I can also see the issue on Russian: https://ru.wikipedia.org/wiki/%D0%A0%D0%BE%D0%B1%D0%B8%D0%BD%D1%81%D0%BE%D0%BD,_%D0%9A%D0%B0%D1%80%D0%BB_(%D0%B2%D0%B0%D0%BB%D0%BB%D0%B8%D0%B9%D1%81%D0%BA%D0%B8%D0%B9_%D1%84%D1%83%D1%82%D0%B1%D0%BE%D0%BB%D0%B8%D1%81%D1%82)

Appears on German Wikipedia as well, confirming that it seems to affect FlaggedRevs: https://de.wikipedia.org/wiki/Kulturgeschichte?safemode=1 for both legacy and parsoid.

This issue also appears on the Arabic Wikipedia and seems to affect all articles.

• Example 1

For now, I will temporarily hide the inline ToC on Arabic Wikipedia until there is an update on this task.

Found the bug, pushing a fix to prod now: Revert "Move post-processing of flaggedrevs views inside FlaggablePageView" (1251172) · Gerrit Code Review

Source of bug described in a comment on https://gerrit.wikimedia.org/r/c/mediawiki/extensions/FlaggedRevs/+/1239378

briefly, the patch switched from using OutputPage::addParserOutput() to running the pipeline directly, and we didn't notice that hidden inside ::addParserOutput()'s call to ::getParserOutputText() is an initialization of default values for the postprocessing options, including injectTOC.

Reports of duplicated ToC appeared again on plwiki. This time, the additional ToC disappears after purging (which suggests an intermittent issue), but the affected pages were parsed well after deploying a fix for this task.

I have reported that as a separate task: T422590: Pages sometimes parsed with seemingly mobile output format

Same on dewiki – multiple users reported the issue mentioned in T421629 (and another issue which seems similar to T420085) – both of these issues could be solved by purging the affected pages but that led to Vector2022 suddenly showing the duplicated TOC. Purging the page a second time seems to solve that issue but sometimes led to the other issues reoccurring.