Firstly an elements ID should always be unique. If your element IDs aren't unique then you would always get conflicting results. Imagine in your case using two different elements with the same ID.
<form>
a: <input type="number" name="a" id="a"><br>
b: <input type="number" name="b" id="b"><br>
<button onclick="add()">Add</button>
</form>
<script>
function add() {
var a = document.getElementById('a').value;
var b = document.getElementById('b').value;
var sum = parseInt(a) + parseInt(b);
alert(sum);
}
</script>