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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