I guess (but I could easily be in the wrong here) that it just hasn't been high on pg's priority list. As akent said there are alternatives out there, but I guess it would be nice to have duplicate submission detection built-in.
I agree. I'd like to see search functionality added too. It just doesn't make a lot of sense, in my eyes, to not have it on a site like this. It would be infinitely useful on so many different levels.
Try: http://www.hnsearch.com/
There is also http://searchyc.com/ but they are currently down.