I don't overly mind as long as there's a good reason to add an additional post. Giving a heads up that should be noticed qualifies.
In theevent where one would want to avoid double-posting...
Well, usually when I have something else to add and that the previous post is mine, I copy/paste the text of my previous post, delete it, and then write a new post. It's what I do when I've done an IC post, then came back to edit it and felt the changes were significant enough to give the people a heads up. It means that whomever might be posting a response in the meantime will likely see my post again before they can submit theirs, and have an opportunity to adjust accordingly.