PHP form code sometimes being executed twice simultaneously

Question

Code similar to this is executed when a form is saved:

$wpdb->query("DELETE FROM Unit_list WHERE group = '$group_id'");
$order = 1;
$unique_units = array_unique($units);
foreach ($unique_units as $unit) {
    $wpdb->insert("Unit_list", array("unit" => $unit, "group" => $group_id, "sort_order" => $order));
    $order++;
}

A few times a day there would be some duplicates.

If the form was submitted twice with enough time for this part of the code to run each time there would be no duplicates because the second time would involve the whole first group being deleted.

For there to be duplicates this code must be running twice at the exact same time.

Double-clicking quickly would do it but if a third click was slow and not simultaneous the group would get deleted and not involve duplicates.

After working out that double clicking was the cause, the solution is to stop subsequent clicks of the form submission button from being detected.

Check the log to see if there are multiple calls to the URL. — Barmar
– Barmar, Commented Jul 14, 2016 at 0:29
The form submission goes back to the same URL when updating a user (sometimes leads to duplicates in the ordered items [an ordered list of units]) but there are different URL parameters when creating users. — Luke Wenke
– Luke Wenke, Commented Jul 14, 2016 at 0:44
@LukeWenke perhaps you don't "think" it's a double-click but you don't know until you read your web server logs. It's all guesswork until you do. Correlate, then confirm or disprove, please. — Michael - sqlbot
– Michael - sqlbot, Commented Jul 14, 2016 at 2:03

Community · Accepted Answer · 2017-05-23 11:58:56Z

1

See a client-side (jQuery) solution here: Prevent double submission of forms in jQuery

And a PHP one here: http://phpsense.com/2006/prevent-duplicate-form-submission/

edited May 23, 2017 at 11:58

CommunityBot

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answered Jul 14, 2016 at 2:34

ldg

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2 Comments

Luke Wenke Over a year ago

Thanks... the code uses a complex $('#editUserForm').submit(function(e){ function that involves return false (for validation problems) or $form.submit();.... It began with e.preventDefault(); but I recently commented that out for some reason...

Luke Wenke Over a year ago

So I just added $('#submit-form-button').prop('disabled', true); before $form.submit();

Luke Wenke · Accepted Answer · 2016-07-14 02:50:03Z

0

It seems the problem is people are clicking multiple times. The php code updating the database runs several times if I click on the submit button several times...

But...

I am unable to duplicate the problem of ordered database entries being interleaved. I suspect it would require the mouse clicks to be within a few milliseconds of each other. Perhaps there is a problem with their mouse or browser causing double click events to fire off. It happens for particular users multiple times over the course of days.

answered Jul 14, 2016 at 2:50

Luke Wenke

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