But discussion there is closed, and discussion here is active. Doesn't that suggest that this should not be killed, and instead should simply be allowed to run its natural course?

I didn't say we would kill the item. We don't close ongoing discussions on duplicate posts.

You can see from this chart[0] that the item disappeared down the rankings at pretty much exactly the same time you said that. So if you didn't kill the item, did you apply a penalty? Doing anything that takes the item off the front page - especially when it's already off the "newest" page - is pretty much equivalent to killing it.

[0] http://hnrankings.info/8124822,8125111/

Of course. We demote duplicates off the front page. That is the whole point about duplicates.

We don't, however, stop an ongoing discussion from continuing. That is a significant difference.