This is a very good catch. I wonder how everyone have missed it until now. I'm not sure if I should call this a bug, though. But I'm pretty sure this is an issue.
In order to reproduce this, you can perform a double click on the button or even hit the ENTER key twice very fast when the cursor is in the tags field, for example.
The simple workaround would be (assuming Q2A v1.6.3):
1. Locate file qa-include/qa-page-ask.php
2. Look for this line:
$qa_content=qa_content_prepare(false, array_keys(qa_category_path($categories, @$in['categoryid'])));
3. Add the following lines after the line found in the previous step:
$qa_content['script_var']['qa_form_submitted'] = 0; // Unable to use boolean values due to qa_js function
$qa_content['body_footer'] = '
<script>
$(document).ready(function(){
$("form[name=' . qa_js('ask') . ']").submit(function(event) {
console.log(qa_form_submitted);
if (qa_form_submitted == 1)
event.preventDefault();
else
qa_form_submitted = 1;
});
});
</script>
';
4. Test it in Chrome, FireFox, Internet Explorer and Safari (if possible)
5. Report back with the results of the test (if it is working or not for each browser)
Note the change doesn't hide the button but rather it just disables the form submission, which I guess it is better as it will also handle submission by hitting the ENTER key twice.