Incomplete javascript example
var has_spoken = [];
on('message', (stuff) => {
if (has_spoken.indexOf(userID) >= 0) {
// not first message
} else {
// is first message
}
});
Your question is a basic programming question.
Not really a TwitchAPI or TMI question.
You just need a way to store/collect the data then see if they have spoken already or not.