PHP MVC刷新表单

I made contact form to contact with administration. This is Controller method code

function sendEmail()
    {
        $this->setView("Index", "ContactForm");
        require_once 'libs/functions.php';
        if(isset($_POST['submit']) && $_POST['submit']=='sendEmail'){
            $headers = 'From: '.sanitizeString($_POST['sender']);
            $to=sanitizeString($_POST['receiver']);
            $subject=sanitizeString($_POST['subject']);
            $message=sanitizeString($_POST['message']);
            $control=mail($to, $subject, $message, $headers);
            if($control>0){
                $this->view->alert = "Sukces";
            }
            else{
                $this->view->alert = "Błąd";
            }
        }
    }

When I submit data and refresh form with message "Sukces" then method send email one more time. In the other forms is the same situation. How can I solve this? I tried unset($_POST) but it doesn't work. I tried header ("Location: url") but then message "Sukces" doesn't appear.

P.S. Sorry for my English :)