I can't duplicate this. I've tried in Firefox 3.5.7, IE8, Chrome, Opera and Safari.

I notice there is already a double-vote check in vote/vote_functions.php, but it doesn't get used because a double vote simply "unvotes" the first vote.

        // get vote history for this post:
        
        $sql = "SELECT vote_rating FROM " . TABLE_POSTVOTES . " WHERE vote_post_id = %d AND vote_user_id = %d AND vote_rating != %d";
        $voted = $h->db->get_var($h->db->prepare($sql, $post_id, $user_id, -999));
        
        if ($voted == $vote_rating) {
            // Repeat vote. Must be from a double-click. Return false and 
            $json_array = array('result'=>$h->lang['vote_already_voted']);
            echo json_encode($json_array);
            return false;
        }

I'll pass this ticket to Carlo :)

No feedback, so I'm moving this ticket to "Test".