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We call those copycat or follow-up posts and mostly downweight them, on the grounds that predictability makes HN less interesting.

If it's just the same theme, and not just another page on the same thing, why not encourage it instead?

Example: reddit.com/r/redditDayOf

Because front page space is the scarcest resource on HN, and there's more information in 30 distinct stories.

The natural place for follow-up links is, of course, the original thread.