Fatal error: Cannot redeclare PHPMailerAutoload() - php

Question

I am using PHPMailer to send emails from my local host.
I have written a function which is supposed to send emails to registered users who have chosen the option to receive them. (i.e. newsletter subscription, etc)
function email_users($subject, $body) {
include('core/db/db_connection.php');
$sql = "SELECT email, first_name FROM `_users` WHERE allow_email = 1";
$query = mysqli_query($dbCon, $sql);
while (($row = mysqli_fetch_assoc($query)) !== false) {
$body = "Hello ". $row['first_name'] . ", <br><br>" . $body;
email($row['email'], $subject, $body);
}
}
The code that is calling the function:
if (isset($_GET['success']) === true && empty($_GET['success']) === true) {
?>
<h3 class="email_success">Emails have been sent</h2>
Go back to the admin page
<?php
} else {
if (empty($_POST) === false) {
if (empty($_POST['subject']) === true) {
$errors[] = 'A message subject is required.';
}
if (empty($_POST['body']) === true) {
$errors[] = 'A body message is required.';
}
if (empty($errors) === false) {
echo output_errors($errors);
} else {
email_users($_POST['subject'], $_POST['body']);
header('Location: email_users.php?success');
exit();
}
}
// generate email form otherwise
Any idea why I'm getting this error?
Fatal error: Cannot redeclare PHPMailerAutoload()
I would also like to point out that even with this error, the function still works and the emails are being sent...
EDIT: As requested, please see below the function using PHPMailer:
function email($user, $subject, $body) {
require 'phpmailer/PHPMailerAutoload.php';
$mail = new PHPMailer;
/* $mail -> Host,username,password and other misc stuff
$mail->Subject = $subject;
$mail->Body = $body;
$mail->AltBody = $body; etc */
}

Answer 1

If you use
require 'phpmailer/PHPMailerAutoload.php';
in your function, but you call the function 2 times, it will redeclare the class. Simply use require_once() instead.
require_once('phpmailer/PHPMailerAutoload.php');

Answer 2

After much testing, the solution I have found is adding the header redirect into the function and removing it from the calling code:
function email_users($subject, $body) {
include('core/db/db_connection.php');
$sql = "SELECT email, first_name FROM `_users` WHERE allow_email = 1";
$query = mysqli_query($dbCon, $sql);
while (($row = mysqli_fetch_assoc($query)) !== false) {
$body = "Hello ". $row['first_name'] . ", <br><br>" . $body;
email($row['email'], $subject, $body);
header('Location: email_users.php?success');
}
}
Also, as pointed out by honerlawd, require_once is needed in order for this to work, otherwise it will send an email only to the first account found in the database. Without redirecting to the email_users.php?success, this will cause an infinite loop, no matter if I call require_once or require.
Would this be the correct approach or is it just a temporary messy fix?