Hash table version:

on !*:JOIN:#: {
  if ($istok($hget(nicknames,$chan),$nick,32)) { .notice $nick Welcome back to $chan $+ , $nick $+ . Glad to see you here again! =] }
  else { .notice $nick Welcome to $chan $+ , $nick $+ . We hope you make this your new hangout spot. }
}
on !*:PART:#: { hadd -m nicknames $chan $addtok($hget(nicknames,$chan),$nick,32) }
on !*:QUIT: {
  var %x = $comchan($nick,0)
  while (%x) {
    hadd -m nicknames $comchan($nick,%x) $addtok($hget(nicknames,$comchan($nick,%x),$nick,32))
    dec %x
  }
}
on *:CONNECT: {
  if (!$hget(nicknames)) { hmake nicknames 100 }
  if ($isfile(nicknames.hsh)) { hload nicknames nicknames.hsh }
}
on me:*:PART:#: { hsave -o nicknames nicknames.hsh }
on *:DISCONNECT: { hsave -o nicknames nicknames.hsh }
on *:EXIT: { hsave -o nicknames nicknames.hsh }