Why are applications duplicated from the site?

Question

PHP

Good evening. Guys, there is a form for sending an application on the site, the script is standard mailto. The problem is that sometimes (1 out of 5) the application is duplicated, as if the script is filled in 2 times. Sometimes even 3 or 4 times. What could be the problem?
PS
After clicking the Submit button, the user is redirected to another page, which eliminates multiple clicks on submitting the form.

<?php
if(isset($_POST['Имя'])) {
        $title = "Заявка";
        $name = $_POST['Имя'];
        $number = $_POST['Номер'];
        $email = $_POST['Почта'];

        $mess = '
        Имя:'.$_POST['Имя'].'
        Номер телефона:'.$_POST['Номер'].'
        E-mail:'.$_POST['Почта'].'
        ';

        $to = '[email protected]';
        $from='[email protected]';
        $mess = iconv("utf-8", "windows-1251", $mess);
        mail($to, $title, $mess, 'From:'.$from);
}
header('Location: /accept.html');
exit;

Answer 1

The site's favicon may be missing.