The "Open in Media Viewer" box appears twice, one on top of the other, when viewed on file description pages using Mozilla Firefox 40.0.2 (the latest version).
It's intermittent. Sometimes there are two pairs; some times one, even on the same page. see https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)#Duplicate_media_viewer_links
No, I normally use MonoBook, but just now I switched to Vector, edited File:First Minister of Scotland, Nicola Sturgeon.jpg and immediately got two pairs of "Open in Media Viewer"/cogwheel link boxes above the "This file is unlicensed for use on Wikipedia" box. Also happened with the next two file description pages that I edited too (also Vector).
We do. Is there a reason you didn't use wikipage.content hook? (It is fired after the page is loaded or previewed, after a page is saved with VE and updated, and when it's previewed with live preview – generally every time when the page content is updated. ''mediawiki.page.image.pagination' fires it too, it seems, which you might also want to hook to.)
I didn't write that code but I would guess the reason is JS hooks are not documented anywhere and which one to use when is only passed down via oral tradition.
Thanks for doing that; I'll file a patch :)
Yes; sorry, the patch was not deployed separately. It will go out to Commons with the regular 1.26wmf21 deployment later today (see https://www.mediawiki.org/wiki/MediaWiki_1.26/Roadmap).