master.xml

<data>
  <the-pip>
    <entry>
       <published-by>
       <item id="191" handle="tim-powell" section-handle="team-members" section-name="Team Members">Tim Powell</item>
      </published-by>
    </entry>
    
    <entry>
       <published-by>
       <item id="192" handle="andy-minton" section-handle="team-members" section-name="Team Members">Andy Minton</item>
      </published-by>
    </entry>
    
    <entry>
       <published-by>
       <item id="191" handle="tim-powell" section-handle="team-members" section-name="Team Members">Tim Powell</item>
      </published-by>
    </entry>
  </the-pip>
</data>

master.xsl

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">

  <xsl:output method="xml" indent="yes" />

  <xsl:key name="staff-by-surname" match="published-by" use="item" />
  
  <xsl:template match="data">
    <hi>The Pip</hi>
    <xsl:apply-templates select="the-pip/entry"/>
    
  </xsl:template>

  <xsl:template match="the-pip/entry">
    <xsl:apply-templates 
    select="published-by[count(. | key('staff-by-surname', item)[1]) = 1]"/>
  </xsl:template>
  
  
  <xsl:template match="published-by/item">
    <p><xsl:value-of select="key('staff-by-surname', item)"/></p>
  </xsl:template>
  
  
</xsl:stylesheet>