The issue here is you are appending the same element over and over again on multiple clicks. You can remove the previously added element before adding a new one like
function Confirm() {
var form = document.forms[0];
var oldInput = document.getElementById('myInput');
if (oldInput !== null) form.removeChild(oldInput);
var confirm_value = document.createElement("INPUT");
confirm_value.setAttribute('id', 'myInput');
confirm_value.type = "hidden";
confirm_value.name = "confirm_value";
if (confirm("Do you want to delete supplier ?")) {
confirm_value.value = "Yes";
} else {
confirm_value.value = "No";
}
form.appendChild(confirm_value);
}