The same node matched by several keys

      <xsl:stylesheet
                  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                  version="2.0">
            <xsl:output  method="xml"
                        indent="yes"
                        omit-xml-declaration="yes"/>
            <xsl:key  name="kkk"
                        match="*"
                        use="@y"/>
            <xsl:key  name="jjj"
                        match="vvv"
                        use="."/>

            <xsl:template  match="/aaa">
                  <ggg>
                        <xsl:apply-templates  select="key('kkk','b')"/>
                  </ggg>
                  <hhh>
                        <xsl:apply-templates  select="key('jjj','Z')"/>
                  </hhh>
            </xsl:template>

            <xsl:template  match="*">
                  <xsl:copy-of  select="."/>
            </xsl:template>

      </xsl:stylesheet>

      <aaa>
            <bbb  y="a">X</bbb>
            <bbb  y="b">Y</bbb>
            <vvv  y="b">Z</vvv>
      </aaa>

      <ggg>
            <bbb  y="b">Y</bbb>
            <vvv  y="b">Z</vvv>
      </ggg>
      <hhh>
            <vvv  y="b">Z</vvv>
      </hhh>