Drupal 7 how to prevent multiple form submissions (server side) - drupal-7

Question

Here is the working module code I am testing with:
/**
* #file myform.module
*/
/**
* Implements hook_menu().
*/
function myform_menu() {
$items['myform'] = array(
'title' => 'myform',
'page callback' => 'drupal_get_form',
'page arguments' => array('myform'),
'access callback' => true,
'type' => MENU_NORMAL_ITEM
);
return $items;
}
/**
* Form
*/
function myform() {
$form['Value'] = array(
'#title' => t('Value'),
'#type' => 'textfield',
'#description' => t('You may not enter the same value twice. (unless you hit enter really fast).'),
'#required' => true,
);
$form['submit'] = array(
'#type' => 'submit',
'#value' => t('Submit')
);
return $form;
}
/**
* Validate
*/
function myform_validate($form, &$form_state) {
if (isset($form_state['values']['Value']) && trim($form_state['values']['Value'])!=''){
// prevent duplicates
db_set_active('test');
$r = db_query("SELECT id FROM test WHERE value = '".$form_state['values']['Value']."'");
$n = $r->rowCount();
if ($n) {
form_set_error('Value', t('This value has already been submitted.'));
}
db_set_active();
}
}
/**
* Submit
*/
function myform_submit($form, &$form_state) {
for ($i=0; $i<=10000000; $i++) {
// do nothing
}
db_set_active('test');
db_insert('test')->fields(array('value'=>$form_state['values']['Value']))->execute();
db_set_active();
}
The validation hook prevents duplicate values from being inserted, unless I hit the enter key or submit button really fast in which case the same value is inserted into the database multiple times.
How do I prevent duplicate values from being inserted?

Answer 1

If you mean users being accidentally clicking the submit button more than once, then you should look into Hide submit button module. You can define it as dependency in your module's INFO file.

Answer 2

I had the exact same problem and managed to fix it using the Locking mechanisms from Drupal
In the validate function I used:
function mymodule_custom_form_validate($form, &$form_state){
if (lock_acquire('your_custom_lock_name')) {
// long operations here
} else {
form_set_error("", t("You submitted this form already."));
}
}
And in the submit function I released the lock:
function mymodule_custom_form_submit($form, &$form_state){
// submit code
lock_release('your_custom_lock_name');
}