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<xsl:template match="datenFuerKunden"> <!-- Didn't need the a: -->
    <Kunde>
        <name>
            <xsl:value-of select="nachname"/>
        </name>
        <xsl:variable name="kId" select="@kundenID"/>
        <xsl:apply-templates select="//datenFuerRechnung[@kundenID=$kId]"/>
    </Kunde>
</xsl:template>

<xsl:template match="datenFuerRechnung">
    <Nr>
        <xsl:value-of select="nummer"/>
    </Nr>
</xsl:template>