Rather than make people post a follow-up to clarify a mistake in a reply, allow them to edit/delete their own replies.
This is already possible. Just go to Post -> Manage -> Comments and edit any of your comments.
Or did you mean for users that aren't logged in?
The intent is to let the comment authors delete / edit their own comments, instead of having to make the blog owner delete/edit the comment. Example- I just accidentally posted a comment on a Moveable Type blog in triplicate, and I went to Post -> Manage -> Comments, and could only see other people's comments on my own blog. There was no way to mange my comments on other people's blogs.