This might seem simplistic, but couldn't this problem be solved with a "related" list?

It's been many moons since I've been to Digg, but it had a notification if you're submitting a post with a similar title/URL. While we're discussing it, it may not hurt to have a "previous discussion" list either.

Opinions, information other relevant factors can change over time, so it may not be so bad to revisit a topic.