An item with the same key has already been added. HELP!!!!!

Question

Gurus
I get this error after I add the first name: Code Below
and xml file below too
 
public XmlDocument AddUsers(string queryXml)
{
try
{
Dictionary<string, string> fields = new Dictionary<string, string>();
for (int i = 1; i < 11; i++)
{
XmlDocument xmlDocument = new XmlDocument();
try
{
xmlDocument.LoadXml(queryXml);
}
catch (Exception)
{
throw new Exception("Error reading query string passed to AddUsers Web Service. It may not be a well formed XML.");
}
fields.Add("FirstName", Util.GetXPath(xmlDocument, "//FirstName" + i.ToString()));
fields.Add("MiddleName", Util.GetXPath(xmlDocument, "//MiddleName" + i.ToString()));
fields.Add("LastName", Util.GetXPath(xmlDocument, "//LastName" + i.ToString()));
fields.Add("DisplayName", Util.GetXPath(xmlDocument, "//DisplayName" + i.ToString()));
fields.Add("LoginName", Util.GetXPath(xmlDocument, "//Login" + i.ToString()));
fields.Add("Password", Util.GetXPath(xmlDocument, "//Password" + i.ToString()));
fields.Add("ArchiveStatus", "False");
Inserter inserter = new Inserter("DOCUMENTS_PEOPLE");
foreach (string key in fields.Keys)
{
inserter[key] = fields[key].ToString();
}
}
XmlDocument xmlReturnDoc = new XmlDocument();
XmlDeclaration xmlDeclaration = xmlReturnDoc.CreateXmlDeclaration("1.0", "utf-8", null);
XmlElement rootNode = xmlReturnDoc.CreateElement("AddSysOptions");
xmlReturnDoc.InsertBefore(xmlDeclaration, xmlReturnDoc.DocumentElement);
XmlNode NewNode = xmlReturnDoc.AppendChild(rootNode);
NewNode.InnerText = "Success";
return xmlReturnDoc;
}
catch (Exception ex)
{
AcXmlClasses.AddLogEntry("Error when calling AddUsers: " + ex.ToString(), 1);
return AcException.GetXmlException(ex);
}
}
<User><FirstName1>fn1</FirstName1><MiddleName1>mn1</MiddleName1><LastName1>ln1</LastName1><DisplayName1>fn1 mn1 ln1</DisplayName1><Login1>AF4073</Login1><Password1>5F4DCC3B5AA765D61D8327DEB882CF99</Password1><FirstName2>fn2</FirstName2><MiddleName2>mn2</MiddleName2><LastName2>ln2</LastName2><DisplayName2>fn2 mn2 ln2</DisplayName2><Login2>AC4070</Login2><Password2>5F4DCC3B5AA765D61D8327DEB882CF99</Password2><FirstName3>fn3</FirstName3><MiddleName3>mn3</MiddleName3><LastName3>ln3</LastName3><DisplayName3>fn3 mn3 ln3</DisplayName3><Login3>AJ4077</Login3><Password3>5F4DCC3B5AA765D61D8327DEB882CF99</Password3><FirstName4>fn4</FirstName4><MiddleName4>mn4</MiddleName4><LastName4>ln4</LastName4><DisplayName4>fn4 mn4 ln4</DisplayName4><Login4>AE4072</Login4><Password4>0CC175B9C0F1B6A831C399E269772661</Password4><FirstName5>fn5</FirstName5><MiddleName5>mn5</MiddleName5><LastName5>ln5</LastName5><DisplayName5>fn5 mn5 ln5</DisplayName5><Login5>AF4073</Login5><Password5>9E3669D19B675BD57058FD4664205D2A</Password5><FirstName6/><MiddleName6/><LastName6/><DisplayName6/><Login6/><Password6/><FirstName7/><MiddleName7/><LastName7/><DisplayName7/><Login7/><Password7/><FirstName8/><MiddleName8/><LastName8/><DisplayName8/><Login8/><Password8/><FirstName9/><MiddleName9/><LastName9/><DisplayName9/><Login9/><Password9/><FirstName10/><MiddleName10/><LastName10/><DisplayName10/><Login10/><Password10/><FirstName11/><MiddleName11/><LastName11/><DisplayName11/><Login11/><Password11/><ArchiveStatus/><ROLES><Role>Student</Role></ROLES></User>