If someone with the nick JACK joined the channel, it would set a variable named %didtrace.JACK to $true so if JACK joins more than one of your channels, you will still only do one trace on JACK instead of one for each channel. The variable is automatically unset in 60 seconds. You can remove it if you want, it's a flood protection.

Code:

on *:JOIN:#: {
  if ($nick != $me) !trace $nick
}