function verifierVote(e) {
    if (
           $F('candidat_1') == $F('candidat_2')
        || $F('candidat_2') == $F('candidat_3')
        || $F('candidat_3') == $F('candidat_1')
    ) {
         
        alert(
                "Vous devez choisir trois personnes différentes.\n"
            +   "You have to choose three different persons."
        );
        Event.stop(e);
        return false;
    } else {
        return true;
    }
}

Event.observe('vote', 'submit', verifierVote);