Google gives more comprehensive results too. But there is a time lag with either. I was mining my bookmarks tonight to find links worth posting, and searched to see if one had been posted before. Search YC indicated it had not been, so I posted it, only to discover to my chagrin that someone else had just submitted it just before I did. (My submission is now deleted, to avoid duplication.) I try to search before I submit, so accurate and timely search is helpful.

I haven't done a lot of submitting myself.. I usually just do "Ask HN" type stuff. Hacker News doesn't automatically tell you that the URL you're submitting has already been submitted? Or was the URL slightly different?

HN automatically upvotes the previous post if you submit an EXACT duplicate URL. But some webpages live under more than one URL, and not everyone submits the canonical URL.