Any threaded discussion carries the risk of different subthreads ending up in the same place. The simplest solution is to just not post twice, and trust that the reader can read the rest of the thread; HN threads usually don't get long enough for good comments to get too buried, and not duplicating comments helps avoid that problem. If there's something slightly different, it may be worth linking to another comment in a different subthread and adding a few sentences to cover the differences. Copying a whole comment is never a good answer, and re-wording it to obscure the fact that it's not saying anything new is also bad. New comments should have something new to say.