如何验证表单以避免空字段和使用javascript提交双重条目

Question

I have a form which i'm using java script to prevent empty fields with onsubmit="return validateForm(); ", now i need to avoid multiple submits, is there a way i can use the same validation to add the feature i need, or how can i make it work, here is my code.

  <form name="myForm"class="form-signup" id ="req-form" action="reg_form.php"  onsubmit="return validateForm(); "  method="post">
        <div class="form-group">
    <label for="fname">First Name:</label><span style="color:red;" id="ferror"> </span>
    <input class="form-control" type="text" name="fname" id="fname"  value="<?php echo "$user_fname";?>">

    <label for="lname">Last Name:</label><span style="color:red;" id="lerror"> </span>
    <input class="form-control" type="text" name="lname" id="lname" value="<?php echo "$user_lname";?>">

     <label for="amount">Amount:</label><span style="color:red;" id="aerror"> </span>
    <input class="form-control" type="text" name="amount" id="amount" placeholder="Amount">

    <label for="cedula">personal id:</label><span style="color:red;" id="cferror"> </span>
    <input class="form-control" type="text" name="cedula" id="cedula" value="<?php echo "$user_cedula";?>">

<label for="cedula">comments:</label><span style="color:red;" id="coferror"> </span>
    <input class="form-control" type="text" name="comments" id="comments" placeholder="comments optional">

 <script>
 function validateForm() {
var x = document.forms["myForm"]["fname"].value;
if (x == null || x == "") {
    document.getElementById("ferror").innerHTML="this is invalid name ";
    return false;
}
var x = document.forms["myForm"]["lname"].value;
if (x == null || x == "") {
     document.getElementById("lerror").innerHTML="this is invalid Last name ";
    return false;
}
var x = document.forms["myForm"]["amount"].value;
if (x < 30 || x == "") {
     document.getElementById("aerror").innerHTML="invalid amount the minimun for loan is 30$ ";
    return false;
}


var x = document.forms["myForm"]["cedula"].value;
if (x == null || x == "") {
    document.getElementById("cerror").innerHTML="this is invalid ID ";
    return false;
}
}
 </script>

this works perfectly but now i need to avoid multiple submits, if field is empty or null it gives the error message and if field has info then allow only one submit