Index: wp-comments-post.php
===================================================================
--- wp-comments-post.php	(revision 12207)
+++ wp-comments-post.php	(working copy)
@@ -73,6 +73,9 @@
 if ( '' == $comment_content )
 	wp_die( __('Error: please type a comment.') );
 
+if ( get_user_by('email', $comment_author_email) ) 
+	wp_die( __('Error: please login before posting a comment.') );
+
 $comment_parent = isset($_POST['comment_parent']) ? absint($_POST['comment_parent']) : 0;
 
 $commentdata = compact('comment_post_ID', 'comment_author', 'comment_author_email', 'comment_author_url', 'comment_content', 'comment_type', 'comment_parent', 'user_ID');