To prevent multiple form submission just generate a unique identifier and store the token as a hidden field in the form. Immediately before form processing, check to see if that token has already been submitted. If it hasn't, you can proceed; if it has, you should generate an error.

While creating the form, use uniqid( ) to get a unique identifier:

<?php
$unique_id = uniqid(microtime(),1);
...
?>

Then, when processing, look for this ID:

$unique_id = $dbh->quote($_GET['unique_id']);
$sth = $dbh->query("SELECT * FROM database WHERE unique_id = $unique_id");
if ($sth->numRows( )) {
    // already submitted, throw an error
} else {
   // act upon the data
}

For numerous reason, users may resubmit a form. Usually it may be a slip-of-the-mouse: double-clicking the Submit button. They may hit their web browser's Back button to edit or recheck
information, but then they re-hit Submit instead of Forward.

So inorder to prevent database from being cluttered duplicate records. By generating a token that's placed in the form, you will uniquely identify that specific instance of the form even if the cookies is disabled. While saveing the form's
data, just store the token alongside it which will allows you to easily check if you already have this form and record the database it belongs to.

Start by adding an extra column to your database table — unique_id — to hold the identifier. When you insert data for a record, add the ID also. By associating the exact row in the database with the form, you can more easily handle a
resubmission.