Stop data inserting into a database twice

I was wondering what methods/preventions other programmers use to stop data being entered twice into a MySQL database when a user refreshes on the same page as a form? Obviouly it happens and I need a good way to stop this.

I call this a golden rule of web programming:

Never ever respond with a body to a POST-request. Always do the work, and then respond with a Location: header to redirect to the updated page so that browser requests it with GET.

This way, refreshing will not do you any harm.

Also, regarding a discussion here in comments. To protect from double posting from, say, accidental doubleclicking the Submit button, store an md5() of your form in a text file, and compare the new form’s md5 to the stored one. If they are equal, you are having a double post.

Process the form, then redirect to the results page. Reload then only redisplays the results page.

Ilya's answer is correct, I just wanted to add a little more than would fit in a comment:

If resubmission is dangerous (going back and submitting again, reloading the result page [if you haven't taken Ilya's advice], etc.) I use a "nonce" to make sure the form can only go through once.

On the form page:

<?php
@session_start(); // make sure there is a session

// store some random string/number
$_SESSION['nonce'] = $nonce = md5('salt'.microtime());
?>
// ... snip ...
<form ... >
<input type="hidden" name="nonce" value="<?php echo $nonce; ?>" />
</form>

In the processing page:

<?php
if (!empty($_POST)) {
@session_start();

// check the nonce
if ($_SESSION['nonce'] != $_POST['nonce']) {
    // some error condition
} else {
    // clear the session nonce
    $_SESSION['nonce'] = null;
}

// continue processing

After the form has been submitted once, it cannot be submitted again, unless the user intentionally fills it out a second time.

To state the obvious (I have not seen it here yet...): Never use GET to post data, always use POST, that way the user at least gets a warning if he or she tries to refresh /re-post the page (at least in Firefox, but I suppose in other browsers as well).

By the way, if you cannot afford to have the same data twice, you should also consider a MySQL solution with a unique key (can be a combination of fields) and:

    INSERT INTO ... ON DUPLICATE KEY UPDATE ...