Duplicate Header received from the Server, using OpenXML C#

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C#

ASP.NET

MS-Word

OpenXML

Hi. I am trying to create multiple .docx files (base on the number of rows in gridview) using 1 template only. It goes through but I am encountering a message saying that "Duplicate Headers received from server"
Here is my code:

TextInfo txtInfo = new CultureInfo("en-US", false).TextInfo;
        string sourceFile = Server.MapPath("~/BankInstruction_CoveringLetter/BankInstruction_CoveringLetter.docx");
            foreach (GridViewRow gvRow in gvBankExport.Rows)
            {
                Label lblBankCode = (Label)gvRow.FindControl("lblBankCode");
                Label lblBankTotal = (Label)gvRow.FindControl("lblBankTotal");
                TextBox txtBankName = (TextBox)gvRow.FindControl("txtBankName");
                TextBox txtAddress = (TextBox)gvRow.FindControl("txtAddress");
                TextBox txtAcctNo = (TextBox)gvRow.FindControl("txtAcctNo");
                TextBox txtCurrencyCode = (TextBox)gvRow.FindControl("txtCurrencyCode");
                TextBox txtOfficerTitle = (TextBox)gvRow.FindControl("txtOfficerTitle");
                TextBox txtFirstName = (TextBox)gvRow.FindControl("txtFirstName");
                TextBox txtLastName = (TextBox)gvRow.FindControl("txtLastName");
                TextBox txtPosition = (TextBox)gvRow.FindControl("txtPosition");
                string signatory1 = drpSignatory1.SelectedItem.Text.Split('|')[0].ToString();
                string signatory1Pos = drpSignatory1.SelectedItem.Text.Split('|')[1].ToString();
                string signatory2 = drpSignatory2.SelectedItem.Text.Split('|')[0].ToString();
                string signatory2Pos = drpSignatory2.SelectedItem.Text.Split('|')[1].ToString();
                string bankcode = lblBankCode.Text;
                string bankname = txtBankName.Text;
                string address = txtAddress.Text;
                string acctno = txtAcctNo.Text;
                string curr = txtCurrencyCode.Text;
                string offtitle = txtOfficerTitle.Text;
                string lastname = txtLastName.Text;
                string firstname = txtFirstName.Text;
                string offposition = txtPosition.Text;
                string banktotal = lblBankTotal.Text;

                string destinationFile = Server.MapPath("~/BankInstruction_CoveringLetter/") + bankcode + "_BankInsCoverLetter" + DateTime.Now.ToString("yyyyMMdd") + ".docx";
                File.Copy(sourceFile, destinationFile, true);
                Dictionary<string, string> keyValues = new Dictionary<string, string>();
                string a = Convert.ToDecimal(Math.Truncate(Convert.ToDouble(banktotal))).ToString();
                string decimalPart = (Convert.ToDecimal(banktotal) - Math.Floor(Convert.ToDecimal(banktotal))).ToString().Replace("0.", "");
                string numberWordValue = txtInfo.ToTitleCase(NumberToWords(Convert.ToInt32(a)).Replace("-", " "));
                keyValues.Add("BankInstructionDate", DateTime.Now.ToString("MMMM dd, yyyy"));
                keyValues.Add("BANKFULLNAME", txtInfo.ToUpper(bankname));
                keyValues.Add("BANKFULLADDRESS", address);
                keyValues.Add("BankFullContactPerson", firstname + " " + lastname);
                keyValues.Add("BankFullContactPerPosition", offposition);
                keyValues.Add("BankOfficerTitle", offtitle);
                keyValues.Add("BankLastNameContactPerson", lastname);
                keyValues.Add("BANKFULLACCOUNTNO", acctno);
                keyValues.Add("TOTALAMOUNTINWORDS", numberWordValue);
                keyValues.Add("NUMBERCENTS", decimalPart);
                keyValues.Add("CURRENCYCODE", curr);
                keyValues.Add("TOTALAMOUNTINDIGITS", banktotal);
                keyValues.Add("Signatory1", signatory1);
                keyValues.Add("Sign1Position", signatory1Pos);
                keyValues.Add("Signatory2", signatory2);
                keyValues.Add("Sign3Positionsss", signatory2Pos); //NAGERROR KASI UNG ORIGINAL NA REPLACEMENT WORD, PERO ETO GUMANA, NOT SURE KUNG ANONG KAARTEHAN ULIT NG C#
                SearchAndReplace(destinationFile, keyValues);
                Process.Start(destinationFile);

                Response.ContentType = ContentType;
                Response.AppendHeader("Content-Disposition", "attachment; filename=" + Path.GetFileName(destinationFile));
                Response.WriteFile(destinationFile);

            }
            Response.End();

public void SearchAndReplace(string document, Dictionary<string,string> toReplace)
   {
       using (WordprocessingDocument wordDoc = WordprocessingDocument.Open(document, true))
       {
           string docText = null;
           using (StreamReader sr = new StreamReader(wordDoc.MainDocumentPart.GetStream()))
           {
               docText = sr.ReadToEnd();
           }

           foreach (KeyValuePair<string, string> item in toReplace)
           {
               Regex regexText = new Regex(item.Key);
               docText = regexText.Replace(docText, item.Value);
           }

           using (StreamWriter sw = new StreamWriter(wordDoc.MainDocumentPart.GetStream(FileMode.Create)))
           {
               sw.Write(docText);

           }
       }

   }