How to excute some code only for the first asp:button click
protected void Button1_Click(object sender, EventArgs e)
{
    if (Session["Clicked"] == null)
        Session["Clicked"] = true;
    else {
        // We already ran this function once, so do other stuff from now on
        ...
        return;
    }

   // Code below this comment will be executed only on the 1st button click
   ...
}