"How" is different. I just tend to remember stuff like this easily, so I know if something's duplicated. A quick search will usually find it, and that often turns up other stuff that I find interesting in its own right. I haven't scripted this, I just remember stuff.
That also feeds back into the "Why." I've found many things of value (to me) on HN because an article that was vaguely interesting prompted a search for an earlier duplicate, and that search turned up related, more interesting material. Having found any duplicates I then put in the comment to help save other people some time because, well, why not?
However, I'm pretty much considering stopping. Although I'm not really bothered about karma, the recent instance of PG linking the ability to vote to one's average karma has made me re-evaluate what I'm doing. Although it's only approximate, clearly "karma" embodies an indication of the behavior PG wants. Flagging duplicates like this drags down my average karma because it rarely gets up-voted and occasionally gets down-voted. That suggests that, in some sense, PG doesn't want me to do it, even if I think it saves people time and helps to cross-connect related discussions.
If you'd like to discuss this more, feel free to email me.
In general, I like starting over with a blank page if enough time has passed that we will have new faces and changed circumstances. Two weeks is not enough, but a blog post from two years ago might be interesting to re-examine with fresh eyes.
However if overall you are getting positive karma, then overall there is a good fit and don't worry about the odd bit of behaviour that gets negative karma. Unless it's something specifically discouraged, I think you should let it slide.
FWIW, I don't think karma reflects utility for something like this. I just saw someone post "[2005]" as a comment on another post, and it got some upmods. Obviously that's helpful in a certain way, but I didn't bother to upmod it myself. That doesn't mean I don't appreciate it.
So... tl;dr; version: Rock on!
P.S. I find the best way to find previous HN threads that I remember is a site-restricted Google search with a few keywords, e.g.,
http://www.google.com/search?q=site%3Anews.ycombinator.com+p...
Way better ~ http://searchyc.com/previously+submitted?sort=by_date
Separately, would it be useful if HN had a sort presentation that listed posts/threads recently commented upon -- perhaps further weighted by post score -- as opposed to just recent comments (where the quantity combined with flatness of presentation quickly overwhelms me)? Of course, one would have trouble finding the new comment within the old thread -- particularly active ones -- unless there was further utility added to locate it within the thread of conversation of such a post. (If it was in response to a parent comment, it would not sort to the top.)
At which point, the current presentation paradigm for the site is starting to change significantly. Representing both a lot of work for PG, and an open-ended question as to the effects upon the community dynamics.
So... perhaps we are better off with what we have. If a page/topic is brought up again after after a significant period of time (more than a few days), it is handled as a new discussion. With thoughtful people like Rider... perhaps citing older posts/threads for further reference.