How to prevent Insert Duplicate record into Database on Page Refersh.

Page_Load()
{
If(!Page.IsPostBack)
{
Session["update"] = Server.UrlEncode(DateTime.Now.Tostring();
}
}

Page_PreRender()
{
ViewState["update"].ToString() = ViewState["update"].Tostring();
}

SaveButton_Click()
{
If(Session["update"].Tostring() == ViewState["update"].Tostring());
{
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Session["update"] = Server.UrlEncode(DateTime.New.Tostring());
}
}