> wrap in
> <xsl:if test="not(preceding-sibling::Row/Cell[6]/Data = 
> Cell[6]/Data"> ...
> </xsl:if>

Or perhaps

<xsl:if test="not(preceding-sibling::Row[1]/Cell[6]/Data = Cell[6]/Data"> 
  ...
</xsl:if>

if you only want to compare with the immediately preceding row.

Regards,

Michael Kay
http://www.saxonica.com/
http://twitter.com/michaelhkay