<xml>
	<add link='a' value='c' />
	<add link='a' value='b' />
	<add link='a' value='c' />
</xml>
<xsl:for-each select="xml/add[@link='a'][not(following::add/@value=@value)]">
	<link rel="stylesheet" href="{@value}" type="text/css" />
</xsl:for-each>