The next problem is that the Hacker News user interface doesn't encourage people to not cause duplicate answers within a short window: you load the full thread and spend 10-15 minutes reading it. At the time you loaded the page there aren't all of these other replies, so you may as well leave the first one.

You then click the reply button and start typing a reply--FWIW, for longer replies this step can matter even more than the previous one--and you are staring at the single now and are given just the empty box for your reply. When you are done you submit, and maybe you then notice it is a duplicate? You might not, based on where the page loads and how much you are paying attention. Even if you do, do you bother to delete it? I would, but I hate the duplication and also know how to delete my comments.