Receving Email again after refereshing the page

<div>
       <asp:Label ID="Label1" runat="server" Text="Label"></asp:Label><br />
       <br />
       <asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" /></div>

protected void Page_Load(object sender, EventArgs e)
   {
       if (!IsPostBack)
       {
           Session["CheckRefresh"] = Server.UrlDecode(System.DateTime.Now.ToString());
       }


   }
   protected void Button1_Click(object sender, EventArgs e)
   {
       if (Session["CheckRefresh"].ToString() == ViewState["CheckRefresh"].ToString())
       {
           Label1.Text = "Hello";
           Session["CheckRefresh"] = Server.UrlDecode(System.DateTime.Now.ToString());
 string str = "your code will save here or call save() method here";

       }
       else
       {
           Label1.Text = "Page Refreshed";
                 }

   }
   protected void Page_PreRender(object sender, EventArgs e)
   {
       ViewState["CheckRefresh"] = Session["CheckRefresh"];
   }