I have never seen any post of mine marked [dead] yet, including when using a private window and not logged in, so I am very dubious about that claim as well. I have had a single one marked [flagged] so far, and I dispute the fairness of that. I think it was a misunderstanding, which is very common.
> A duplicate means a copy
On HN, if someone submits an article on Tuesday for some news that broke on Monday that received a substantial of comments, it's a dupe.
(You're also not supposed to continually spam your own work here. There's a reason why most of your submissions are autokilled to [dead].)