People want upvotes so they submit it. Setting up scanning for recent articles is probably harder than it seems. Can’t think of any other forum that does it.
The vast majority of dupes posted here are the same domain, site and title. Catching that would be as easy as a call to the Algolia API. I'm using the Hacker News enhancement suite add-on for Firefox and it does that to generate a list of prior threads. That leaves edge cases, but human curation should be enough for that.
Then again, HN checks for canonical URLs, it could check for canonical title, author and excerpt text in metadata as well. But just a bare title match would catch almost everything, especially during high velocity periods. Then again again, I think I mentioned this in the past and dang said they tried to find more robust solutions in software but false negatives and edge cases made it infeasible.
HN could prompt submitters to reply to an existing thread instead of posting a duplicate, if an open thread exists, while allowing the option to post. That should remove the possibility of the software rejecting a legit post due to a bad match. Having the process be entirely automated would probably be a bad idea.
At least now I know what I'm going to do this weekend.
Shows stories shared between the 2 sites.