Empecher insertion double apres rafraichissement

 if (!empty ($_POST['submit']) AND $_POST['submit'] == 'Sauvegarder')
{

 $name = mysqli_real_escape_string($conn,$_POST['nom']);
$username=mysqli_real_escape_string($conn, $_POST['username']);
$mail=mysqli_real_escape_string($conn, $_POST['mail']);  
$password=$_POST['password'];
$role = mysqli_real_escape_string($conn,$_POST['role']);			  

   $sql = "INSERT INTO utilisateur VALUES ('', '".$name."', '".$username."', '".$password."',  '".$role."', '".$mail."')";
		 $res = mysqli_query($conn, $sql);
 echo '<p style="text-align:center">Un utilisateur a ete ajoute avec succes !</p>';
}
 else
	 {
   echo $confirm='';
}


<form action="" method="post"  name="form" id="form" class="form"  onSubmit="return verify(this.password, this.password2);"   > 
<p style="margin-left:5%;" >
 </br>

<label>Nom <em>*</em></label>

<input type="text" name="nom" onkeyup="verif_name(this);" required /></br></br>
 <label>Email <em>*</em></label>

<input type="email" name="mail" required /></br></br>
<label>Nom utilisateur <em>*</em></label>

<input type="text" name="username" required /></br></br>
                        
  <label >Password <em>*</em></label>
      <input type="password"   name= "password" id= "password"  onclick="montre('blabla')" required></br>

<label>Role</label>
<select name="role">
<option>Administrateur UPAG</option>
<option>Doyen</option>
<option>Etudiant</option>
</select></br></br>
			
			
</p>

 <hr style="height: 0px; color: gray; width: 100%; border: 1px solid ;" >

<p style="margin-left:70px;color:black;"> 

<input type="submit" id="submit" name="submit" value="Sauvegarder" class="bouton" style="background-color:green;color:white;font-weight:bold" disabled="true" onclick="var aze=confirm('Voulez-vous inserer cet nouvel utilisateur ?');return aze;" />

</p>

 </form>