How to find out if Xml contains two or more equal elem

Hi,

I have only one XML. and there <Nid> contains same data like 000817441. I
want to throw an email for same data.

How to find out if Xml contains two or more equal element name value with
xsl? But my code is not responding

*Below is XML:*

<?xml version="1.0" encoding="UTF-8"?>
<objects>
  <Contact>
    <Id>003j000001Cj6l8AAB</Id>
    <Nid>000817441</Nid>
  </Contact>
  <Contact>
    <Id>007j000001Cj6l8AAB</Id>
    <Nid>000817441</Nid>
  </Contact>
  <Contact>
    <Id>003j700001Cj6l9AAB</Id>
    <Nid>001191237</Nid>
  </Contact>
  <Contact>
    <Id>003j000801Cj6lAAAR</Id>
    <Nid>010978087</Nid>
  </Contact>
  <Contact>
    <Id>003j000001Cj9l8AAB</Id>
    <Nid>000817441</Nid>
  </Contact>
</objects>


*Below in XSL:*

<xsl:stylesheet version="2.0" xmlns:xsl="http://www.w3.org/
1999/XSL/Transform">
    <xsl:output method="xml" indent="yes" omit-xml-declaration="no"/>
    <xsl:strip-space elements="*"/>
    <xsl:template match="objects">
        <xsl:if
            test="boolean(//objects/Contact[./Nid &gt;
preceding-sibling::node() or ./Nid &gt; following-sibling::Nid])">
            <Nid>
                <xsl:value-of select="Nid"/>
            </Nid>
        </xsl:if>
    </xsl:template>
</xsl:stylesheet>

*Expected output:*

<Nid>000817441</Nid>