you guys honestly think this is 7.7 material? its a pretty good first script but its not that useful, its not that innovative..

ON *:JOIN:#: { 
  if ($address($nick,3) != $address($me,3)) {
    if ($($+(%,xnick,$nick,$chan),2)) {
      .msg $chan Welcome back $nick
      unset $+(%xnick,$nick,$chan)
      halt
    }
    msg $chan Welcome $nick 
  }
}
ON *:PART:#: set -u120 $+(%xnick,$nick,$chan) 1

you forgot the $chan, so if someone left one channel you were on and joined another it would say welcome back regardless.