[xsl] Ignoring a duplicate

I cannot get the choice to work: it does not copy either <Item> and if I 
negate the test
<xsl:when test=" not(preceding-sibling::Item[1] eq .)"/>
it copies both. What am I doing wrong in the test?
Thanks,
Mark

<List>
 <Item>
   <Heading entry="subject" data="Aleš, Mikoláš. 1852-1913">
     <ToRef xref="see" data="Artists - Aleš, Mikoláš. 1852-1913" file="a" 
block="A55-2"/>
   </Heading>
 </Item>
 <Item>
   <Heading entry="subject" data="Aleš, Mikoláš. 1852-1913">
     <ToRef xref="see" data="Artists - Aleš, Mikoláš. 1852-1913" file="a" 
block="A55-2"/>
   </Heading>
 </Item>
<List>

<xsl:template match="Item">
  <xsl:if test="descendant::ToRef">
    <xsl:choose>
      <xsl:when test=" preceding-sibling::Item[1] eq ."/> <!-- Ignore the 
second (and further) identical sequential Items -->
      <xsl:otherwise>
        <xsl:copy-of select="."/>
      </xsl:otherwise>
    </xsl:choose>
  </xsl:if>
..... <!-- lots of other transformations going on -->
</xsl:template> 


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