In the UI that checks for add-on updates when Fx is upgraded to a new version, we call addon.findUpdates twice for add-ons to be updated; once at https://hg.mozilla.org/mozilla-central/annotate/5f3776437298/toolkit/mozapps/extensions/content/update.js#l170 for every add-on, and again at https://hg.mozilla.org/mozilla-central/annotate/5f3776437298/toolkit/mozapps/extensions/content/update.js#l259 for those that we decide need to be updated. Since this call involves a network request, we should do it at most once per add-on.

I suspect this crept in during the change from old to new API in bug 557956, where the old em.update(... nsIExtensionManager.UPDATE_SYNC_COMPATIBILITY...) was changed to forall(addons) addon.findUpdates() at https://hg.mozilla.org/mozilla-central/diff/ab6e93fb7fec/toolkit/mozapps/extensions/content/update.js#l1.179, and then later changes added back the compatibility sync via AddonRepository.repopulateCache() but kept the ...findUpdates().

No, we always used to do the update check twice because IIRC the old API could only return compatibility or update information not both. We were just in such a rush to get Firefox 4 done that we didn't fix this up tthen.

The code described above was re-done in bug 1353194