gistfile1.php

add_filter( 'preprocess_comment', 'baw_no_double_thread' );
function baw_no_double_thread( $comment )
{
	if ( current_user_can( 'administrator' ) ) {
		return $comment;
	}
	global $wpdb;
	if ( ! $comment['comment_parent'] ) {
		$c = $wpdb->get_var( $wpdb->prepare( 'SELECT comment_ID FROM '.$wpdb->comments.' WHERE comment_post_ID = %d AND comment_author_email = %s AND comment_parent = 0', $comment->comment_post_ID, $comment['comment_author_email'] ) );
		if ( (int) $c > 0 ) {
			$comment['comment_parent'] = $c;
		}
	}
	return $comment;
}