Why does the javascript variable name only print once in this html? [duplicate]

When I execute the following code, why does the variable name which is assigned to output only print the first time I use it…not every time I used it?

 <!DOCTYPE html>
<html>
<head>
<title>Test</title>
<script>
window.onload = function(){
    var name = prompt("What's your name?");
    var lengthOfName = name.length

    document.getElementById('output').innerHTML = name;
};
</script>

</body>
</head>
<body>

<h1>My First Heading</h1> <p id="output" </p>
<p>My first paragraph.</p> <p id="output" </p>
<p>My second paragraph.</p>
<p id="output" </p>

</body>
</html>

when i run it…this is the what it looks like

My First Heading dodo bird

My first paragraph.

My second paragraph.

I would expect it to look like this

My First Heading dodo bird

My first paragraph. dodo bird

My second paragraph. dodo bird