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if HN cares that much about not re-posting, HN should make it easier to check. A lot of people don't even know about searchyc. Can't a URL check against searchyc for a high percentage match and give an intermediate "are you sure you want to post this?"

It has had a fair few mentions on this site:

http://www.google.ca/search?q=www.jwz.org%2Fdoc%2Fgroupware....

Some filtering or a more refined search might give the actual (much smaller) number of posts.

I only see one actual post, some 875 days ago: http://news.ycombinator.com/item?id=37824

Another way is to lurk for a few months before posting.