Avoid forwarding mail when reloading the page

The problem is as follows, when you reload the page $intentos=0; is again set to 0 so you will always enter the if. You could use session variables.

This is how they are defined:

<?php
    //Inicias la sesion
    session_start();
    //Compruebas si se ha definido la variable

    if (!isset($_SESSION['count'])) {
        $_SESSION['count'] = 0; // si no lo esta la dejas a 0
    } else {
        $_SESSION['count']++; // si ya existe sumas uno
    }
?>


Si lo aplicamos a tu codigo deberia quedarse algo asi:
//Inicias la sesion
session_start();
//Compruebas si se ha definido la variable
if (!isset($_SESSION['enviado']) || $_SESSION['enviado'] == false) {
  $_SESSION['enviado'] = true; // si no lo esta la dejas a 0
  $mail->send();
  echo 'Enviado';
  return true;
} else {
  echo 'Error';
  return false;
}

I have changed it to a Boolean variable because you do not need to know how many times it has been sent, but if it has already been sent.

EDITION

Seeing that you have to create a new table in the database, I indicate the way I would do it.

Note: I do not know the structure of your database, so this solution will have to be adapted

Let's say you have a user table (to which you send the notification).

This table could be like this:

USUID   USUNOMBRE   USUAPELLIDO USUAPELLIDO2

Then another notification table, with the id of the notification and its message.

Something like this:

NOTIID  NOTIDESC

Finally you create a table of Emails, this could have a date of shipment and the id of the user and notifications (if users have more than one email you could also add the email field) and to ensure a Boolean field to tell us if it has been sent successfully or not.

It would stay like this:

EMAILID USUID   NOTIID  FECENVIO    ENVIADO

That would be the part of the database, as in the first example I explain how to do it with the session variable now I will do it (without complicating a lot with the sql) with the database.

The first thing you should do is to collect in variables the information you need, that is, the id of the user to whom the notification is sent, the id of the notification and the most important thing and what you are interested in, if has been sent.

With a sql more or less like this you could check if that notification has been sent.

SELECT USUID, NOTIID, ENVIADO FROM EMAIL WHERE USUID = ? and NOTIID = ? and ENVIADO = TRUE

So, if you get any registration, it will be that the email has already been sent and also successfully, in the case that you do not have any registration you should do a $mail->send(); and add a new record to the email table in the case that you never would have tried to send that email or an update in the case that the previous shipment was FALSE .

Finally I indicate how to send the email and check if the shipment was correct or not:

//Antes de realizar este bloque de código, debes comprobar si existe una
//fila con los datos que vas ha enviar y el campo enviado en true.
//Si existe, no debe ejecutar el bloque ya que el email ya ha sido enviado.
if(!$mail->send()) { //Intenta enviar el correo
  //Si da error enviandolo
  //SQL INSERT (campo enviado false)      
} else {
  //Si se envia correctamente
  //Comprobar si tiene el campo enviado en false
  if($enviado == false){
     //SQL (campo enviado true)
  }else{
     //SQL INSERT (campo enviado true)
  }
}

Without the code, we can only give you opinions. In your case I would try to make a variable = 0. At the moment that the client wants to send the mail (or when it is time), the variable is set to 1. Then it would put the function that sends it within a:

IF (variable == 1 ){
  enviaMail();
}

When the page is refreshed the variable returns to 0 and the mail is not sent.