For example:
I submitted this about an hour ago:
https://news.ycombinator.com/item?id=3906712
which appeared in the Wall Street Journal
A comment labeled "Dupe" 15 minutes later linked to this:
https://news.ycombinator.com/item?id=39058633
which appeared on frontofficesports.com
The content of the two pieces overlaps in many areas but there is content unique to each one that would otherwise be unavailable if HN readers assumed "Dupe" = identical.
I don't think there is an official policy (anymore), but people on the internet gonna' internet.
My guess is that whenever you submit "breaking news from mainstream media" there's a good chance of story duplication because submitting "breaking news from mainstream media" is so easy and therefore is low-effort for everyone else, too.
Good luck.
I dug out your submission, and yes, I‘d consider it a dupe. That doesn‘t mean you did anything bad. Just that it‘s duplicate.