When you submit a duplicate URL it sends you to the original submissions page instead

It would be more interesting if news.ycombinator.com also checked if a URL was directly linked from a previously submitted URL; the URL was from the same site as the previously submitted URL.

Before you could commit a new submission news.ycombinator.com would present a list of matches (in descending order by date) so that you could compare what you are about to submit versus what is already submitted.