Unique entries from substrings

I am trying to get all unique month-date entries based on a list of
mm/dd/yyyy entries.

I am able to get the unique mm/dd/yyyy entries but I would like to only
get the unique months per year.

I have included a stripped down XML I am reading data from and my test
XSL that should give me the unique values.  From the given data set the
transformation returns now:

"07-2005";"05-2005";"05-2005";

...but what I would like to achieve is:

"07-2005";"05-2005";


I guess the formatting of the date does not work as I expect it.  Is
there another way to find unique entries on a different format than
found in the XML?

Strange to me is that if the formatting would not work, I should see
four entries instead of three.  So something is not working as I think
it should, but I am not able to figure it out.

What do I do wrong????

The XML:

<REPORT_ACCESS>
  <REPORTS>
    <REPORT>
      <!-- not unique in list -->
      <DATE>07/08/2005</DATE>
    </REPORT>
    <REPORT>
      <!-- unique -->
      <DATE>05/08/2005</DATE>
    </REPORT>
    <REPORT>
      <!-- not unique in list -->
      <DATE>07/08/2005</DATE>
    </REPORT>
    <REPORT>
      <!-- unique 05-2005-->
      <DATE>05/09/2005</DATE>
    </REPORT>
    <REPORT>
      <!-- unique 07-2005-->
      <DATE>07/09/2005</DATE>
    </REPORT>
  </REPORTS>
</REPORT_ACCESS>

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

DATE[not(concat(substring(../preceding-sibling::REPORT/DATE,1,2),'-',sub
string(../preceding-sibling::REPORT/DATE,7,4)) =
concat(substring(.,1,2),'-',substring(.,7,4)))]"/>
  <!--

-->
  <xsl:template match="/REPORT_ACCESS">
    <!-- for testing
    <xsl:copy-of select="$unique-dates-month"/>
    -->
    <xsl:call-template name="build-list-month-year">
      <!-- if used with unformatted list, it works fine as expected -->
      <xsl:with-param name="list" select="$unique-dates-formatted"/>
    </xsl:call-template>
  </xsl:template>
  <!--

  -->
  <xsl:template name="build-list-month-year">
    <xsl:param name="list"/>
    <xsl:for-each select="$list">
      <xsl:text>"</xsl:text>
      <xsl:value-of
select="concat(substring(.,1,2),'-',substring(.,7,4))"/>
      <xsl:text>";</xsl:text>
    </xsl:for-each>
  </xsl:template>
</xsl:stylesheet>

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