I've seen this a few times recently, and wanted to check what the 'official' line is. My understanding is that when you want to expand on an answer that someone else has already given, the way things are intended to work is that you edit the existing answer rather than create a whole new one for yourself.
Is this correct? If so then why don't people do it more often, and how can we encourage them to do so?
The entirely depends on what you're doing, why you're doing it and how nice you want to be.
If you are merely adding a point and aren't going to change the intent or purpose of the original answer, it's cleaner and better for the site to add the point to the other answer.
However, you are free (by the license and site policy) to take existing answers and improve upon them in your own separate answer provided you credit the original poster in your answer.
If the post is just upgrading a single answer, I think it should be an edit. But I've seen a lot of posts made that expand on an answer and several others. These posts are more like digests of the entire thread, comments included. I think they're worth posting on their own.
