[dang]

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

[forgotmypw3]

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

[dang]

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.