I think you're confused about what "new" means in HN submissions. Asking people is unnecessary. The software knows when the story was submitted.
Reference to dups provides proof that "top" stories are being missed, since one dup might have 100+ upvotes, while another will have just a few upvotes and no comment saying it's a dup; meaning to me the only difference is the volume of comments/votes between the two.
Beyond that, I've seen research that shows seeding a submission with even just a few comments/upvotes greatly increases the odds of future traction.
Point is to reduce noise to signal ratio.
You want to ask users if a story is new, so that they check a timestamp manually and maybe even report correctly?
That's inane.