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related posts / similar discussions
There's some overlap in various discussions on this board (or busy forums in general).
A related post/similar discussions add-on might be useful.
#1 - for the passive reader (list recent post with similar words in the title or post)
#2 - for the discussion-starter (before starting a new thread the propsed post is compared to existing threads)
#3 - for the active reader (others can suggest related links/threads, pref. with some weight depending on user level)