How to prevent duplicate value in PDO

Question

I want to restrict duplicate value, How could I do.

if($_GET["data"] == "add_class"){

            $name = $_POST["txtName"];
            $sql = "Insert into tbl_class (class_name) values (:class_name);";
            $insert = $conn->prepare($sql);
            $insert->bindParam(':class_name', $name);

            if($insert->execute()){
                   echo json_encode("Insert Success");}
            else{ 
                   echo json_encode("Insert Faild");}    
    }