Earlier quoted context omitted.
You can resubmit a link after a year, I think? In any event, there's no way to block resubmissions programatically: the site owner could just change the name of the file.
The page content's hash or any other blueprint (an arbitrary regex match) could be checked against for resubmissions. For convenience, if resubmission should be detected, the original post could be topped on the frontpage nevertheless, because old material may as well be worth revisiting for several reasons -- I for one had missed this article before, and enjoyed it reading greatly now.
page content hash
Change one byte. regex match
Now we get into ship-of-theseus arguments. What counts as the "same" article? 99 percent? 51 percent?What if you resubmit a blog post that's picked up 150 more comments since it was last posted? How would HN tell the comments apart from the article text?
What if someone submits a blog post that heavily quotes another article that's been submitted, in the process of rebutting it, and ends up being a 80% match?
What if you include the full text of moby dick, in a display:none ? What if the page loads the text of the article with javascript? Would the HN submissionbot have to render CSS and javascript?