Thanks for that detailed comment. This was a rare instance when I didn't search first before posting, because I figured the duplicate detector would just upvote a previous thread if there was one. But I see the duplicate detector still allows new threads in some cases--I'm not sure which cases.
A case I saw a while ago in which the duplicate detector missed a duplicate submission because the URL was not identical resulted in a long, active thread. That was explicitly approved by pg in a reply to the new thread, who noted that the old thread had happened quite a while ago. So I guess I'm still learning both about technical performance of the duplicate detector and curator review of duplicate threads that get posted anyway. But, thanks, I appreciate the reminder to search for previous submissions before posting here.
http://news.ycombinator.com/item?id=411043
That one is now closed for comments, but perhaps it's worth reading them before commenting here and repeating stuff.