> > <xsl:template match="user">
> >  <xsl:if test="preceding-sibling::user/@uid=@uid">
> >   <xsl:message>oops</xsl:message>
> >  </xsl:if>

If the XML is large you could use a key:

<xsl:key name="uids" match="user" use="@uid"/>

And then count how many <user>s have the current <user>'s @uid:

<xsl:template match="user">
  <xsl:if test="count(key('uids', @uid) > 1))">
	<xsl:message>oops<xsl:message>


cheers
andrew