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