"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.