Avoid execution of code while refresh Page

Hi , here we have to discuss the how to avoid the execution of code While you refresh the page.. suppose if you have written the code for sending email to users.. after clicking the button the code has been executed suppose if you , Refresh the same page once again the Email sent to the user so here we have to avoid the execution on code on page_Refresh
This is code for C#.NET
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());
}
else
{
Label1.Text = "Page Refreshed";
}
}
protected void Page_PreRender(object sender, EventArgs e)
{
ViewState["CheckRefresh"] = Session["CheckRefresh"];
}