Node test problem with preceding-sibling

Hi:

I am trying to build a list of unique entries but somehow I am stuck
with the node test. Seems like I am missing here some basic
understanding.  I tried lots of combination but I am not able to figure
out the correct expression.

The problematic part is the following:

<xsl:variable name="unique-dates"
  select="/REPORT_ACCESS/REPORTS/REPORT
  /DATE[not(preceding-sibling::DATE = .)]"
  />

I am trying to get all unique dates.  Currently I get the following
output:
Unique dates:
05/31/2005
05/30/2005
05/31/2005
05/30/2005

What I would expect is:
Unique dates:
05/31/2005
05/30/2005

I know this is a trivial question, but currently I am stuck and I would
really appreciate some friendly push in the right direction.

Thanks in advance,  Andreas

My test XSL:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

<xsl:output method="text" encoding="UTF-8" indent="no"/>

<xsl:variable name="unique-dates"
  select="/REPORT_ACCESS/REPORTS/REPORT
  /DATE[not(preceding-sibling::DATE = .)]"
  />

<xsl:template match="REPORT_ACCESS">
  <xsl:apply-templates select="REPORTS"></xsl:apply-templates>
</xsl:template>

<xsl:template match="REPORTS">
  <xsl:message>Reports found</xsl:message>
  <!--
  <xsl:apply-templates select="REPORT"></xsl:apply-templates>
  -->

  <xsl:for-each select="$unique-dates">
    <xsl:message><xsl:value-of select="."/></xsl:message>
  </xsl:for-each>

  </xsl:template>

<xsl:template match="REPORT">
  <xsl:value-of select="DATE"></xsl:value-of>
  <xsl:text> - </xsl:text>
  <xsl:value-of select="preceding-sibling::REPORT/DATE"/>
  <xsl:text> || </xsl:text>
</xsl:template>

</xsl:stylesheet>

... and my test XML:

<REPORT_ACCESS>
  <REPORTS>
    <REPORT>
      <REPORT_ID>755600996</REPORT_ID>
      <NAME>Monthly Performance Summary</NAME>
      <DESCRIPTION>Phone Bill Summary::Monthly Performance
Summary</DESCRIPTION>
      <DISPLAY_TEXT>Monthly Performance Summary</DISPLAY_TEXT>
      <DATE>05/31/2005</DATE>
      <VERSION>01/09/2005</VERSION>
      <BATCH_BILL_START_DT>01/02/2005</BATCH_BILL_START_DT>
      <BATCH_BILL_END_DT>01/09/2005</BATCH_BILL_END_DT>
      <EFFECTIVE_DATE>05/31/2005</EFFECTIVE_DATE>
      <EXPIRATION_DATE>05/22/2012</EXPIRATION_DATE>
      <CLIENT_ID>364</CLIENT_ID>
      <CIC_ID>903</CIC_ID>
      <MODE>Premium</MODE>
    </REPORT>
    <REPORT>
      <REPORT_ID>546871065</REPORT_ID>
      <NAME>Monthly Performance Summary</NAME>
      <DESCRIPTION>Phone Bill Summary::Monthly Performance
Summary</DESCRIPTION>
      <DISPLAY_TEXT>Monthly Performance Summary</DISPLAY_TEXT>
      <DATE>05/30/2005</DATE>
      <VERSION>01/16/2005</VERSION>
      <BATCH_BILL_START_DT>01/09/2005</BATCH_BILL_START_DT>
      <BATCH_BILL_END_DT>01/16/2005</BATCH_BILL_END_DT>
      <EFFECTIVE_DATE>05/31/2005</EFFECTIVE_DATE>
      <EXPIRATION_DATE>05/22/2012</EXPIRATION_DATE>
      <CLIENT_ID>364</CLIENT_ID>
      <CIC_ID>903</CIC_ID>
      <MODE>Premium</MODE>
    </REPORT>
    <REPORT>
      <REPORT_ID>563132464</REPORT_ID>
      <NAME>Monthly Performance Summary</NAME>
      <DESCRIPTION>Phone Bill Summary::Monthly Performance
Summary</DESCRIPTION>
      <DISPLAY_TEXT>Monthly Performance Summary</DISPLAY_TEXT>
      <DATE>05/31/2005</DATE>
      <VERSION>01/23/2005</VERSION>
      <BATCH_BILL_START_DT>01/16/2005</BATCH_BILL_START_DT>
      <BATCH_BILL_END_DT>01/23/2005</BATCH_BILL_END_DT>
      <EFFECTIVE_DATE>05/31/2005</EFFECTIVE_DATE>
      <EXPIRATION_DATE>05/22/2012</EXPIRATION_DATE>
      <CLIENT_ID>364</CLIENT_ID>
      <CIC_ID>903</CIC_ID>
      <MODE>Premium</MODE>
    </REPORT>
    <REPORT>
      <REPORT_ID>136207414</REPORT_ID>
      <NAME>Monthly Performance Summary</NAME>
      <DESCRIPTION>Phone Bill Summary::Monthly Performance
Summary</DESCRIPTION>
      <DISPLAY_TEXT>Monthly Performance Summary</DISPLAY_TEXT>
      <DATE>05/30/2005</DATE>
      <VERSION>01/30/2005</VERSION>
      <BATCH_BILL_START_DT>01/23/2005</BATCH_BILL_START_DT>
      <BATCH_BILL_END_DT>01/30/2005</BATCH_BILL_END_DT>
      <EFFECTIVE_DATE>05/31/2005</EFFECTIVE_DATE>
      <EXPIRATION_DATE>05/22/2012</EXPIRATION_DATE>
      <CLIENT_ID>364</CLIENT_ID>
      <CIC_ID>903</CIC_ID>
      <MODE>Premium</MODE>
    </REPORT>
  </REPORTS>
</REPORT_ACCESS>

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