mb_send_mail() sends 2 same mails on chrome - php

Question

My php mail form sends 2 same mails each time.
I'm new to php. I've tried things I can do, but couldn't find the solution by myself so far...
PHP version is 5.2.17
It somehow works right on IE, Firefox and Safari on Windows 8.
Use mail() instead of mb_send_mail(), but it was the same.
The contents of the mails I received from the form seems OK.
Uploaded on another server, but was the same.
Simple code without any other code, with very simple HTML also didn't work properly.
example: mb_send_mail( "[email address]", "Test", "It's a test mail", "From:[email address]" );
Here's my code from the form.
form.html
<form action="./confirm.php" method="post">
xxxxxxxxxxxxxxxxxxxxxxxx
<input type="submit" value="Confirm">
</form>
*Uses Angular to validate.
confirm.php
<?php
session_start();
if(!$_POST){
header('Location: ./form.html');
}
foreach ($_POST as $key => $value) {
${$key} = $value;
$_SESSION[$key] = $value;
}
?>
<form action="./send.php" method="post">
xxxxxxxxxxxxxxxxxxxxxxxx
<input type="submit" name="submit" value="Send">
</form>
send.php
session_start();
$email_address = "xxxxx#gmail.com";
$message = "xxxxxxxxxxxxxxxxxxxxxxx";
$message .= "xxxxxxxxxxxxxxxxxxxxxxx";
mb_language("ja");
mb_internal_encoding("UTF-8");
$header = "From:".$email_address."\n";
$header .= "Reply-to:".$email_address."\n";
$header .= "Content-Type: text/plain\n";
$header .= "X-Mailer:PHP/". phpversion();
mb_send_mail($_SESSION['email'],"subjecct",$message,"From:$email_address");
session_destroy();
Why is my code sending each mail twice, and how can I fix it?

Answer 1

After session_destroy();, add some debug marks to check if this PHP runs twice.
e.g. file_put_contents('debug.txt', file_get_contents('debug.txt')+1);
It can help.

Answer 2

Thanks #zairwolf for your advice to add a debug line.
Now it's working correctly.
Not sure which made solve it because I've tried few things to fix it.
I think it could be one of these:
Added session token which is like:
confirm.php
$token = sha1(uniqid(mt_rand(), true));
send.php
$key = array_search($_POST['token'], $_SESSION['token']);
if ($_SESSION['xxxxx']=="" || $key == false) {
header('Location: ./error.php');
}
Separate "thankyou.php" from "send.php"
I put both "sending program" and "showing thank you html" in "send.php" actually. So noticed might should separate it somehow.