If you put this in the Alt+D aliases window instead of Alt+R remotes, you don't need the 'alias' keyword.

Code:
alias chatter {
  if (!$0) { echo -a syntax: /chatter NICK | return }
  if ($chat($1)) { window -a = $+ $1 | echo -ag chat window already open to $1 | return }
  dcc chat $1
}


https://en.wikichip.org/wiki/mirc/identifiers/$chat