<c>
   <d>aa</d>
   <d>ab</d>
</c>

<a>
  <b>
    <x>aa01</x>
  </b>
  <b>
    <x>aa01</x>
  </b>
  <b>
    <x>aa02</x>
  </b>
  <b>
    <x>ab01</x>
  </b>
</a>

<?xml version="1.0" encoding="UTF-8"?><xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
    <xsl:template match="/">
        <c>
                    <xsl:for-each select="//a/b[not(x=preceding-sibling::b/x)]">
                        <d>
                                <xsl:value-of select="substring(x,1,2)"/>
                        </d>
                    </xsl:for-each>
        </c>
    </xsl:template>
</xsl:stylesheet>