Before becoming active on SlickDeals, I was a heavy poster on Woot's community deals section. It's very similar to SD, but went to rubbish when Amazon bought them. Spam started taking over too. Anyway, their deals section had this built in automatic quick search for the link you're trying to post up that would activate right as you click the "post" button. So I find a deal I want to post, I paste the link into their website as usual, type in my personal tips/strings attached/promo codes/etc and then click post. At that time, the message board runs this quick little search for other post with an identical URL or maybe several keywords? If it finds one, it stops the post and says "Are you sure this is not a duplicate topic?" and links you to the post it found. From there, you can view the link and then either cancel your post or push it through anyway. It would seem that this would cut back on the number of duplicate deal posts we have here.
You can tell users to "search before you post" all you want. But users still won't. Secondly, I've tried using the search bar to search for the deal I'm about to post and it's not the best.... It pulls up some of what I'm looking for and a lot of random stuff. I end up not seeing my deal posted, so I post it, then have 3 users curse me out for duplicate posting.