I commented out the following lines (494-503, at least in WP 2.9.1) in the “wp-includes\comments.php” file:
// Simple duplicate check
// expected_slashed ($comment_post_ID, $comment_author, $comment_author_email, $comment_content)
// $dupe = "SELECT comment_ID FROM $wpdb->comments WHERE comment_post_ID = '$comment_post_ID' AND comment_approved != 'trash' AND ( comment_author = '$comment_author' ";
// if ( $comment_author_email )
// $dupe .= "OR comment_author_email = '$comment_author_email' ";
// $dupe .= ") AND comment_content = '$comment_content' LIMIT 1";
// if ( $wpdb->get_var($dupe) ) {
// if ( defined('DOING_AJAX') )
// die( __('Duplicate comment detected; it looks as though you’ve already said that!') );
//
// wp_die( __('Duplicate comment detected; it looks as though you’ve already said that!') );
// }