The validation script is blurring the field multiple times causing the repeated error. This should fix it:
<script>
var postDelay = false;
function doUniqueCheck(licenseNumber) {
$.post("process2.php" , {licenseNumber:licenseNumber} , function(data){
if(data != "")
alert(data);
});
}
function isUnique(licenseNumber){
if (postDelay) clearTimeout(postDelay);
postDelay = setTimeout('doUniqueCheck("'+licenseNumber+'")',200);
}
</script>