XSLT - Remove duplicates from mapped results - xslt

Question

This isn't quite the threads on removing duplicates I've found on this forum.
I have a key/value map and I want to remove duplicates from the final results of the mapping.
Source Document:
<article>
<subject code="T020-060"/>
<subject code="T020-010"/>
<subject code="T090"/>
</article>
Mapping:
<xsl:variable name="topicalMap">
<topic MapCode="T020-060">Value 1</topic>
<topic MapCode="T020-010">Value 1</topic>
<topic MapCode="T090">Value 3</topic>
</xsl:variable>
Desired Result:
<article>
<topic>Value 1</topic>
<topic>Value 3</topic>
</article>
XSLT I'm working with (note, it has a testing tags and code to make sure the mapping works):
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2.0">
<xsl:output method="xml" encoding="utf8" indent="yes" exclude-result-prefixes="#all"/>
<xsl:template match="article">
<article>
<xsl:for-each-group select="subject" group-by="$topicalMap/topic[#MapCode = #code]">
<test-group>
<code>Current code: <xsl:value-of select="#code"/></code>
<topic>Current keyword: <xsl:value-of
select="$topicalMap/topic[#MapCode = #code]"/></topic>
</test-group>
</xsl:for-each-group>
<simple-mapping><xsl:apply-templates/></simple-mapping>
</article>
</xsl:template>
<!-- Simple Mapping Topics -->
<xsl:template match="subject">
<xsl:variable name="ArticleCode" select="#code"/>
<topic>
<xsl:value-of select="$topicalMap/topic[#MapCode = $ArticleCode]"/>
</topic>
</xsl:template>
<!-- Keyword Map -->
<xsl:variable name="topicalMap">
<topic MapCode="T020-060">Value 1</topic>
<topic MapCode="T020-010">Value 1</topic>
<topic MapCode="T090">Value 3</topic>
</xsl:variable>
</xsl:stylesheet>
Doing the group-by that way produces nothing. If I duplicate the topics in the source document and do group-by="#code" that works to remove before applying the mapping. But I want to remove resultant duplicate values not duplicate keys.
Simple-mapping stuff is just to show working code.

Answer 1

Use
<xsl:for-each-group select="subject" group-by="$topicalMap/topic[#MapCode = current()/#code]">
<topic>
<xsl:value-of select="current-grouping-key()"/>
</topic>
</xsl:for-each-group>
or better yet
<xsl:key name="map" match="topic" use="#MapCode"/>
<xsl:template match="article">
<article>
<xsl:for-each-group select="subject" group-by="key('map', #code, $topicalMap)">
<topic>
<xsl:value-of select="current-grouping-key()"/>
</topic>
</xsl:for-each-group>
</article>
</xsl:template>