[xsl] Variable containing unique values

I've read the book and looked at the examples but I can't manage to
create a variable containing unique values for my xml. All I get is a
list of all values.
I need a variable containing unique values for rolloverDate. I've tried
all sorts of rules for testing for current rolloverDate not being equal
to previous rollover dates but I can't get it to work.
Can you help please.
Regards
        Jan Hesselberth

The code I have is

        <xsl:variable name="unique-dates">
                <xsl:for-each
                select
="REPORT/Rollover/RolloverForecast/summaryAccount/element">
                        <xsl:sort
select="/rolloverDate[not(preceding::rolloverDate= current())]"/>
                <xsl:copy-of select="."/>
                </xsl:for-each>
        </xsl:variable> 

The xml snippet looks like

<summaryAccount id="12345"> 
        <isAutomatedRollover>N</isAutomatedRollover> 
        <element id="1"> 
                <rolloverDate>2008-04-04</rolloverDate> 
        </element> 
        <element id="2"> 
                <rolloverDate>2008-04-04</rolloverDate> 
        </element> 
</summaryAccount> 
<summaryAccount id="12346"> 
        <isAutomatedRollover>Y</isAutomatedRollover> 
        <element id="1"> 
                <rolloverDate>2008-04-22</rolloverDate> 
        </element> 
</summaryAccount> 
<summaryAccount id="12347"> 
        <isAutomatedRollover>Y</isAutomatedRollover> 
        <element id="1"> 
                <rolloverDate>2008-04-22</rolloverDate> 
        </element> 
        <element id="2"> 
                <rolloverDate>2008-04-17</rolloverDate> 
        </element> 
</summaryAccount>



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