sorting and grouping

I've got a sorting problem. My XML is as follows:

<person>
	<name>Kermit</name>
	<birthday>3</birthday>
	<birthmonth>January</birthmonth>
</person>
etc. 

XSLT:

<xsl:for-each select="person[birthmonth='January']">
    <xsl:sort select="birthday" data-type="number"/>
    <xsl:sort select="name"/>
    <xsl:value-of select="birthday"/>
    <xsl:text> - </xsl:text>
    <xsl:value-of select="name"/>
    <br/>
</xsl:for-each>

With the above code, each birthday number is repeated. What I would like is
to have the birthday number appear only once followed by the names of people
whose birthday that is. I suspect it can be done by testing if the birthday
value is the same as the preceding sibling, but I can't get it right. Does
anyone have any idea how to solve this? 

Mick