Code:
on 1:OPEN:=:{ 
  if ($NickChats($nick) >= 2) echo You already have a chat window with $nick !
}

Alias NickChats {
  var %total = $chat(0)
  var %c = 0
  while (%total) {
    if ($1 == $chat(%total)) inc %c
    dec %total
  }
  return %c
}

mIRC Chm Help 6.16.0.3 Full Anchored!