Please check email as well!

Since Gravatar images are linked to the provided email address, even if the imposter uses a (slightly) different nickname, they can APPEAR to be the administrator (or any user) if they enter that user’s email address when posting a comment.

Could you modify your plugin to compare email addresses with registered users as well, and give an appropriate error if a match is found?

Thanks!

http://wordpress.org/extend/plugins/duplicate-comment-authors/