Prevent multiple form submission using Javascript

@onatm : Using same id won't help. Only first instance of tag will be handled by JS, not the ones after that.

You can do something like this....Use jquery. Give both buttons the same class say "SUBMIT". Now write the following code:

var myForm = document.forms[0];
$(document).ready(function(){
    $(".SUBMIT").click(function(){
        if(validateInput()){
            $(".SUBMIT").attr("disabled", true).val("Submitting..");
            var url = "any URL you like";
            var data = {
                           field1: myForm.field1.value,
                           field2: myForm.field2.value,
                           .
                           .
                       };
            $.post(url, data,
                function(Result){
                    alert(Result); // Write whatever logic you want
                }
            )
            .complete(function(){
               $(".SUBMIT").attr("disabled", false).val("Submit");
            })
            .error(function(){
               alert("Some error");
               $(".SUBMIT").attr("disabled", false).val("Submit"); 
            });
        }
        else{
            alert("Enter the missing fields first"); // Write whatever your logic
        }
    });
});

function validateInput(){
    if (all fields are entered) // Write you logic here
        return true;
    else
        return false
}

The code should solve your first problem. For second problem, you have to explicitly write code to test authenticity for each field in the function validateInput().

onsubmit="
    document.getElementById('myButtonTOP').disabled=true;
    document.getElementById('myButtonTOP').value='Submitting, please wait...';
    document.getElementById('myButtonBOTTOM').disabled=true;
    document.getElementById('myButtonBOTTOM').value='Submitting, please wait...';
    return true;"

Would be better to call a javascript function declared in a <script></script> region of course.

So i would write a function like...

var mySubmitted = false;

function MyCheckSubmit()
{
    if (mySubmitted)
        return false; // Already submitted!

    if (!validateInquiryForm())
        return false; // Validation failed!

    // disable submit buttons

    document.getElementById('myButtonTOP').disabled=true;
    document.getElementById('myButtonTOP').value='Submitting, please wait...';
    document.getElementById('myButtonBOTTOM').disabled=true;
    document.getElementById('myButtonBOTTOM').value='Submitting, please wait...';

    // Store in our global variable that we submitted this form.
    mySubmitted = true;

    return true; // Ok, we can submit!
}

and of course in the form

onsubmit="return MyCheckSubmit();"

This will also work if you press multiple time "enter" key in an input textbox, since it uses an internal variable to check if form was already submitted.

Be careful to declare the script block of course before the form, possibly in the head section.

you could place this in correct condition when the form validation conditions are met, thus only disabling on submit, you would need to return 'this' as reference in your onsubmit="return MyCheckSubmit(this);"

  var topButton = document.MyInquiry.submitinquiryform; // top
  var botomButton = document.otherMyInquiry.submitinquiryform; // bottom

 if(beingSubmitted.name == "MyInquiry"){ 

   topButton.value = "Please wait...";
   //beingSubmitted.action = "process.php" // action page
   beingSubmitted.submit();

  }else{
   //beingSubmitted.action = "process2.php" // action page
    botomButton.value = "Please wait...";
    beingSubmitted.submit();
}
//beingSubmitted.action = "process.php" // action page would handle for both form you could check which form is being submitted by sending a value in a hidden filed 
 topButton.disabled = true;
 botomButton.disabled = true; 

I noticed you have no action eather on the form