PHP if else statement issue with !=

i cant figure out what is the mistake.

    if((!isset($_SESSION['user_username'])) || (!isset($_SESSION['user_role']))){
       header("location:index.php");
    }elseif( $_SESSION['user_role'] != "admin" || $_SESSION['user_role'] != "superadmin" ){
       header("location: noaccess.php");
    }

This is my code.... Even if i log in with admin user (user_role=admin), it is redirecting to noaccess.php