Designer News
Where the design community meets.
6 years ago from Cristian Moisei, http://meet-cristian.com
Unless there's a bug in DN right now, it already does that.
If it detects a URL that already has been posted, it will say "This story has already been posted x days ago, are you sure you want post it?" The confirmation button will then be labeled "I'm sure, post it."
It seems to me that it's more users that are asshats.
Designer News
Where the design community meets.
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On the case of duplicated URLs, it could be detected while filling the "New Story" form. About self promotion articles, well... a voting down button may do the trick.