append cloned node to same xml document

Am trying to append a clone node lets say

<Property Id="3" Name="Deadline"></Property>

into the same document with class name "AlphaCertificationsIndividual" but the compiler gives me this error: The node to be inserted is from a different document context

<Root>
  <Class Name="ECMInstruction" Style="Top">
    <Entity Id="1" Name="DocumentInformation" />
    <Entity Id="2" Name="CustomerInformation" />
    <Property Id="1" Name="DocumentTitle">
    </Property>
    <Property Id="2" Name="DateCreated">
      <Lists>
        <ListName>ws_Users</ListName>
        <ListName>dfdfdfd</ListName>
      </Lists>
    </Property>
    <Property Id="3" Name="Deadline">
    </Property>
  </Class>
  <Class Name="AlphaCertificationsIndividual" Style="Top">
    <Entity Id="1" Name="DocumentInformation" />
    <Property Id="1" Name="DocumentTitle">
    </Property>
    <Property Id="2" Name="DateCreated">
      <Lists>
        <ListName>ws_Users</ListName>
        <ListName>dfdfdfd</ListName>
      </Lists>
    </Property>
    <Property Id="3" Name="Deadline">
    </Property>
  </Class>
</Root>

the code am using:

    XmlDocument xmldoc = new XmlDocument();
    xmldoc.Load("sample.xml");
    foreach (string id in properties)
    {
        XmlNode props = xmldoc.DocumentElement.SelectSingleNode("//Class[@Name='" + curClass + "']/Property[@Id='" + id + "']");

        XmlNode cloneNode = props.CloneNode(true);

        foreach (var item in dcList.SelectedItems)
        {
            XmlNodeList classes = commonMethods.LoadDocument(xml).DocumentElement.SelectNodes("//Class[@Name='" + item + "']/Property[last()]");
            foreach (XmlNode c in classes)
            {
                String propertyid = c.Attributes["Id"].Value.ToString();
                int.TryParse(propertyid, out value);
                value = value + 1;
                cloneNode.Attributes["Id"].Value = value.ToString();
                c.ParentNode.AppendChild(xmldoc.ImportNode(cloneNode,true));
                xmldoc.Save("sample.xml");
            }
        }
    }