Yes, I've seen stories marked as dupe. But aren't they posts pointing to exactly the same URL or with exactly the same title? That isn't sufficient IMHO. It has to detect posts about the same news subject. It's not a trivial problem, as follow-up stories in the media could be detected as dupes, rightly or wrongly, depending on the criteria of what exactly is a dupe.
There are tons of dupes that don't get killed.
May I see an example of this? If it's a dupe, but not killed, there is a likely chance someone vouched for it.