I set about eliminating duplicate lines (where I have defined a
duplicate line as one where the first title element matches that of some
other line.

   <xsl:template match="line[title[1] eq
preceding-sibling::line[1]/title[1]]"/>
   <xsl:template match="line">
     <xsl:apply-templates select="title[1]"/>
   </xsl:template>

But this is not eliminating the duplicates.

Now I know there are other ways of doing this but I'm interested in
finding the flaw in the code above.

  <xsl:template match="line[title[1] = preceding-sibling::line/title[1]]"/>
  <xsl:template match="line">
    <xsl:apply-templates select="title[1]"/>
  </xsl:template>