XSLT Bookmarks

I am new to this so, please, forgive me if I ask anything that should 
be basic knowledge.  I have this:
 
    <xsl:for-each select="/PriceDiscrepancy/Customer">
    <xsl:variable name="bookmark">
      <xsl:value-of select="CustomerNumber"/>
    </xsl:variable>
    <fox:outline internal-destination="{$bookmark}">
    <fox:label><xsl:value-of select="CustomerNumber"/></fox:label>
    </fox:outline>
    </xsl:for-each>
 
 
to create bookmarks in my pdf.  Problem is, there are multiple records 
that have the same "CustomerNumber" so I get a bookmark for each line.  
How can I get one bookmark per customer and not one for each record?
 
Thanks in advance!
 
Ken