The Favorites feature is handy for marking specific questions as important for some reason. What if we made it possible to use that signal in the duplicate close dialog so that favorited candidates (for the current search, or the default tag+most-linked search) show up earlier and with a star somewhere? This is similar to Boost duplicate post search results by incoming link count, but using individual favorites instead of overall link count. It's also similar to Improve positioning of favorites in your search results, but more tightly scoped.
That way, finding a canonical you've seen before would be a lot easier in many cases: just make sure you favorite it, once, and subsequent searches get the benefit automatically instead of requiring you to dig through favorites or browser bookmarks for the right canonical.