12.04 using xfce + lxdm, rapid login will lose xfce4-notifyd pid

Hi, all:
    I am using xfce + lxdm(autologin), I need a function is re-login the X, but lxdm logout without restart X, so I made a script:
        #!/bin/bash
        sudo service lxdm restart &

    this script can restart X, and autologin again, but when I test this script, it some times show this message:
        Unable to start notification daemon
        Another notification xndaemon is already running

    it seems xfce4-notifyd is missing on ps aux(so you can not know it`s pid now, but it sure is running), and if use command:
        notify-send <some word>
    you can once again find the xfce4-notifyd`s pid by "ps aux | grep xfce4-notifyd".

    is it a bug of 12.04? since 10.04 not using xfce4-notifyd I won`t get this problem on 10.04.