c# - for ループ内での SQL クエリの実行

Question

for (int i = 0; i <= GridView1.Rows.Count - 1; i++)
{
    string toemail = GridView1.Rows[i].Cells[2].Text;
    string FID1 = GridView1.Rows[i].Cells[0].Text;
    GridViewRow row = GridView1.Rows[i];
    CheckBox Ckbox = (CheckBox)row.FindControl("CheckBoxMark1");
    if (Ckbox.Checked == true)
    {
        sendMail(toemail);

        //ClientScript.RegisterStartupScript(Page.GetType(), "validation", "<script language='javascript'>alert('Email send Succesfully')</script>");
        ClientScript.RegisterStartupScript(Page.GetType(), "validation", "<script language='javascript'>alert('Email sent Succesfully on " + test + "')</script>");

        cn1.Open();
       //cmd4.CommandText = "Insert into TrackingFaculty_det  (EmailsentDate)  values (@EmailsentDate) WHERE FID=@FID";
       cmd4.CommandText = "update TrackingFaculty_det SET EmailsentDate=@Email WHERE FID=@FID  ";
       cmd4.CommandType = CommandType.Text;
       cmd4.Connection = cn1;

       cmd4.Parameters.Add("@Email", SqlDbType.DateTime, 8);
       cmd4.Parameters["@Email"].Value = sdt;
       cmd4.Parameters.Add("@FID",SqlDbType.VarChar,10);
       cmd4.Parameters["@FID"].Value = FID1;
       cmd4.ExecuteNonQuery();
       cn1.Close();
    }
}

//これはc#コードの一部であり、asp.net

for loopが実行されるたびに実行するupdation必要があるdatabase場合に問題が発生しますが、クエリをfor loop複数回実行するとエラーが発生します。これに対する解決策はありますか?updateThe variable name '@Email' has already been declared. Variable names must be unique within a query batch or stored procedure.