The example you linked to uses the following code:
document.getElementById("result").innerHTML += event.data + "<br>";
So, it appends the new event data to the content of the result element. To replace it, just change it to
document.getElementById("result").innerHTML = event.data + "<br>";
In short, your question doesn't have anything to do with how you produce the event at server-side, but everything to do with how you consume the event, in the browser.