Hello,

First of all, change the file:
send-mail.php

from:
$mail_result = mail( $to, $subject, $body, $headers );

to:
$mail_result = wp_mail( $to, $subject, $body, $headers );

otherwise SMTP will not work.

On my personal email though, I receive 2 same emails each time. Anyone knows how to fix this?

Thanks for your time and please fix this bug. It was a happy release with email notification fixed, but smtp should work fine as well.