Help with XML

i need help with this bit of code, any help will be greatly appreaciated...

private void btnWriteXml_Click(object sender, EventArgs e)
{
    XmlTextWriter xtw = null;
    string fileName = "ComboItemName.xml";
    try

{

xtw = new XmlTextWriter(fileName, Encoding.UTF8);
xtw.Formatting = Formatting.Indented;
xtw.WriteStartDocument();
xtw.WriteStartElement("ITEMS");

So this method only adds the current text in the text boxes and re-writes anything else...

xtw.WriteElementString("NAME", this.txtName.Text.ToString());
xtw.WriteElementString("URL", this.txtUrl.Text.ToString());

this method works but i need help with a way to check to see if the items in the list box exits in the XML file, otherwise it duplicates averything...

for (int i = 0; i < this.comboBoxUrl.Items.Count; i++)
{
xtw.WriteElementString("NAME", this.comboBoxUrl.Items[i].ToString());
xtw.WriteElementString("URL", this.comboChannels.Items[i].ToString());
}
for (int j = 0; j < this.comboBoxUrl.Items.Count; j++)
{
 xtw.WriteElementString("URL", this.comboBoxUrl.Items[j].ToString());
}

xtw.WriteEndElement();

xtw.WriteEndDocument();

}

finally

{

if (xtw != null)

xtw.Close();

}

}