Most of the time, randomers on duel servers know exactly where they are and what they are doing : they're here to ruin people's fun for their own childish little pleasure...So put "duel" as a password for duel servers won't serve as a warning as those randomers are already aware of where they set foot.
I'd rather have a password put on this forum in a specific thread for each server, or even by asking the admin of the server. A bit elitist, but if that's the cost for real duels without been interrupted, I'm ready to accept it...