>>262-266
はいはいワロスワロスwww

<xsl:for-each select="a">
<xsl:sort select="b" order="decending" />
<xsl:if test="not(preceding-sibling::a[b = current()/b])">
<!-- あとはご自由にどうぞ -->
</xsl:if>
</xsl:for-each>