// declare a global variable on page
    private Boolean IsPageRefresh = false;
    protected void Page_Load(object sender, EventArgs e)
    {        
        if (!IsPostBack)
        {
            ViewState["postids"] = System.Guid.NewGuid().ToString();
            Session["postid"] = ViewState["postids"].ToString();
            TextBox1.Text = "Hi";

        }
        else
        {
            if (ViewState["postids"].ToString() != Session["postid"].ToString())
            {
                IsPageRefresh = true;
            }
            Session["postid"] = System.Guid.NewGuid().ToString();
            ViewState["postids"] = Session["postid"];
        }
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        if (!IsPageRefresh) // check that page is not refreshed by browser.
        {
            TextBox2.Text = TextBox1.Text + "@";

        }
    }