Problem when openning excel document. Two popups come up.

The following code I have too explorers popup for saving excel files.

What am I missing.

protected void Page_Load(object sender, EventArgs e)
    {
        DataTable dt = (DataTable)Session["EXPORT_RESULT"];
        string filename = "export.csv";
        filename = string.Format("filename={0}", filename);
        Response.AppendHeader("content-disposition", filename);
        Response.ContentType = "application/vnd.ms-excel";
        Response.ContentEncoding = System.Text.Encoding.Default;

        // Envoyer les données (fichier CSV) au Browser
        Response.Write(GetCSVString(dt, false));
    }