XML checking for duplicates before saving, if duplicate found tell me about it

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C#

XML

Hi Guys/Gals,

struggeling a little here (this is all still really new to me so baby language);

I want to check for duplicates and if I try and enter in the same value a message box comes up telling me, I want to check APIKEY and VCODE. If they match values already in my XML I don't want it to add again.

<?xml version="1.0" encoding="utf-8"?>
<APISAVE xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <ID>3e9316bc-6190-4d1d-81e1-9130755ee0ad</ID>
  <APIKEY>1234</APIKEY>
  <VCODE>1234</VCODE>
</APISAVE><?xml version="1.0" encoding="utf-8"?>
<APISAVE xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <ID>c67ade2a-d568-40d2-a72f-29f896903fd1</ID>
  <APIKEY>1234567</APIKEY>
  <VCODE>1234567</VCODE>
</APISAVE><?xml version="1.0" encoding="utf-8"?>
<APISAVE xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <ID>12e777f2-910e-443f-ab6d-b1d2053ae12b</ID>
  <APIKEY>1234</APIKEY>
  <VCODE>1234</VCODE>
</APISAVE>

My Code looks like this

APISAVE.cs:

public class APISAVE
    {
        static void main(string[] args)
        {
            XmlDocument xmldoc = new XmlDocument();
            xmldoc.Load("data.XML");
            XmlNodeList userNodes = xmldoc.SelectNodes("data.XML");
            foreach (XmlNode userNode in userNodes) ;

        }

        private string id;
        private string APIkey;
        private string VCode;

        public string ID
        {
             get { return id; }
             set { id = Guid.NewGuid().ToString(); }
        }

        public string APIKEY
        {
            get { return APIkey; }
            set { APIkey = value; }
        }

        public string VCODE
        {
            get { return VCode; }
            set { VCode = value; }

        }

        public static void SaveData(object obj, string Filename)
        {

            XmlSerializer sr = new XmlSerializer(obj.GetType());
            TextWriter writer = new StreamWriter(Filename, true);
            sr.Serialize(writer, obj);
            writer.Close();

        }
    }

Here is my Button:

private void button1_Click(object sender, EventArgs e)
{
    {
        try
        {


            APISAVE info = new APISAVE();
            info.APIKEY = txtAPI.Text;
            info.VCODE = txtVerC.Text;
            info.ID = info.ID;
            APISAVE.SaveData(info, "data.XML");

        }

        catch (Exception ex)

        {
            MessageBox.Show(ex.Message);
        }
    }
}

my end goal is to use the data put in here later on.