Sometimes there are two (or more) threads for what is essentially the same story. When this happens, Dan (or one of the other moderators) frequently chooses one thread to keep, and copies the comments from the thread(s) to be deleted into the thread to be kept. If commenters have already posted the same comment to both threads, this requires more manual cleanup by the moderators. There's a lot of moderation that happens behind the scenes here to keep the site as usable as it is.

As nkurz said. See also:

https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...

https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...