how to avoid duplicate values in XML using asp.net?

// Open the XML doc  
XmlDocument myXmlDocument = new XmlDocument();
myXmlDocument.Load(Server.MapPath("AddProducts.xml")); 
XmlNode myXmlNode = myXmlDocument.DocumentElement.FirstChild; 

// Create new XML element and populate its attributes  
XmlElement myXmlElement = myXmlDocument.CreateElement("product");
myXmlElement.SetAttribute("name", Server.HtmlEncode(txtname.Text)); 

// Insert data into the XML doc and save  
myXmlDocument.DocumentElement.InsertBefore(myXmlElement, myXmlNode);
myXmlDocument.Save(Server.MapPath("AddProducts.xml"));        

My XML file is:

<?xml version="1.0" encoding="utf-8"?>
<Footballers>
    <product name="iop" />
    <product name="Selvit E" />
    <product name="Selvit E" />
    <product name="Ventrifort A" />
    <product name="Generator" />
    <product name="TEst" />        
    <product name="test" />
    <product name="Test" />
    <product name="test" />
    <product name="ILT" />
    <product name="No Parking" />
    <product name="No Parking" />
    <product name="Livol Liquid" />
    <product name="G-promin" />
</Footballers>

In my xml file stores duplicate values. I don't want to store duplicate values. How can i do this?