How to ignore messages from commands?

Create a field:

Code:

ArrayList<Player> onDuty = new ArrayList<Player>();

The /onduty command:

Code:

// Make sure command wasn't sent by console
if(sender instanceof Player) {
    Player p = (Player) sender;
    // If player's an op and is not currently on duty
    if(p.isOp() && !onDuty.contains(p)) {
        // Put the player on duty
         onDuty.add(p);
    }
}

And your /offduty command:

Code:

if(sender instanceof Player) {
    Player p = (Player) sender;
    // If player's an op and is currently on duty
    if(p.isOp() && onDuty.contains(p)) {
        // Put the player off duty
         onDuty.remove(p);
    }
}