Avoid Executing twice Server-Side Click Event of a Button

Please let me know...

Best way of avoiding execution of Button_Click Event on server-side for more than one time.

Currently I'm using following approach:

//added a page prerender event to initialize Update Object
protected void Page_PreRender(object obj, EventArgs e)
{
 ViewState["update"] = Session["update"];
}


//page Load event
protected void Page_Load(object sender, EventArgs e)
{

 if (!Page.IsPostBack)
 {
  //initialize session object
  Session["update"] = Server.UrlEncode(System.DateTime.Now.ToString());
 }

}


//Button Event
protected void btnSave_Click(object sender, EventArgs e)
{
 if (Session["update"].ToString() == ViewState["update"].ToString())
 {
  // Save button Code
 }
}

The code works great. But here we're creating a Session Object here; there must be other ways to achieve this objective.

Just Curious