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.
Possible implementations: #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)