PHP Mailer sending duplicate emails – PHP Versions

So as the title implies
Im trying to send an email (one email) but the problem I’m facing is bizarre.
Whenever debug is turned on, email is sent only once.
But when debug is turned off, around 3 to 4 emails are sent at once.

NOTE: I’m using localhost not an actual server.

to diagnose the problem, I did the following:
1- used a md5 to generate random string in the "subject" to check whether email is sent with same contents or different. and the results were totally different. meaning, emails weren’t duplicate but actually being sent couple of times.
2- opened the project in a browser with no extensions to make sure the problem wasn’t in an extension loading my project page more than once. and the results were also similar to number 1, different "subject" also.

so, long story shot. i have no idea what causes this problem to happen. and why it only stop happening when debug is turned on.

NOTE: this is not my first time to use PHP mailer, but my first time to face this problem. I’m also using the latest version of PHP mailer (6.5.1)

Here is my entire code:
in PHP mailer file:

<?php
use PHPMailerPHPMailerPHPMailer;
use PHPMailerPHPMailerException;
use PHPMailerPHPMailerSMTP;

require ABSPATH.'inc/phpmailer/script/src/PHPMailer.php';
require ABSPATH.'inc/phpmailer/script/src/Exception.php';
require ABSPATH.'inc/phpmailer/script/src/SMTP.php';



function SendEmail(){
    $mail = new PHPMailer(true);
    try {
        //Server settings
        $mail->SMTPDebug = SMTP::DEBUG_OFF;
        $mail->SMTPAuth = true;
        $mail->SMTPSecure = 'ssl';
        $mail->isSMTP();
        $mail->Host       = '*****';
        $mail->SMTPAuth   = true;
        $mail->Username   = '*****'; 
        $mail->Password   = '*****';
        $mail->SMTPSecure = PHPMailer::ENCRYPTION_SMTPS;
        $mail->Port       = 465;
        $mail->SMTPAutoTLS = false;
        $mail->SMTPOptions = array(
            'ssl' => array(
                'verify_peer' => false,
                'verify_peer_name' => false,
                'allow_self_signed' => true
            )
        );
        $mail->ClearAllRecipients();
        $mail->clearAttachments();
        //Recipients
        $mail->setFrom('*****', '******');
        $mail->addAddress('*******', '*****');
        $mail->addReplyTo('******', '******');
    
        //Attachments
        $mail->addAttachment(ABSPATH.'upload/dog.jpg', 'new.jpg');
    
        //Content
        $mail->isHTML(true);
        $mail->Subject = md5(rand()); //This is how I'm checking whether it sends same email with same subject header or different ones. and it does send different ones
        $mail->Body    = 'This is the HTML message body <b>in bold!</b>'. md5(rand());
        $mail->AltBody    = "To view the message, please use an HTML compatible email viewer!";
        $mail->ContentType = 'text/html; charset=utf-8rn';
        $mail->send();
        echo 'Message has been sent';
    } catch (Exception $e) {
        echo "Message could not be sent. Mailer Error: {$mail->ErrorInfo}";
    }
}

and here is the file I’m using to call the function

<?PHP
// NO LOOP here
require_once(ABSPATH.'inc/phpmailer/phpmailer.php');
SendEmail();