Une solution

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform(...)">

<xsl:template match="/">
<xsl:for-each select="/personnes/personne">
<xsl:sort select="@nom" />
<xsl:variable name="currentLetter" select="substring(@nom, 1, 1)" />
<xsl:variable name="thisNode" select="." />
<xsl:for-each select="/personnes/personne[starts-with(@nom, $currentLetter)][1]">
<xsl:sort select="@nom" />
<xsl:if test="generate-id(.)=generate-id($thisNode)">
<h1><xsl:value-of select="$currentLetter" /></h1>
</xsl:if>
</xsl:for-each>
<p><xsl:value-of select="@nom" /></p>
</xsl:for-each>
</xsl:template>

</xsl:stylesheet>