Xslt gouping issue - xslt

Question

I have an xml like below
<XYZ>
<Name>abc</Name>
<Year>1984</Year>
<Model>qwe</Model>
<Name>abc</Name>
<Year>1987</Year>
<Model>qwe</Model>
<Name>qweqr</Name>
<Year>1977</Year>
<Model>tryet</Model>
</XYZ>
Which needs to be transformed to
<XYZ>
<Name>abc</Name>
<Year>1984</Year>
<Year>1987</Year>
<Model>qwe</Model>
<Name>qweqr</Name>
<Year>1977</Year>
<Model>tryet</Model>
</XYZ>
Here Name can be considered as key

Answer 1

With XSLT 1.0, this can be done using Muenchian Grouping with following XSLT:
<?xml version="1.0" encoding="UTF-8" ?>
<xsl:transform xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
<xsl:output method="xml" omit-xml-declaration="yes" encoding="UTF-8" indent="yes" />
<xsl:strip-space elements="*"/>
<xsl:key name="names" match="Name" use="text()"/>
<xsl:key name="models" match="Model" use="text()"/>
<xsl:template match="XYZ">
<XYZ>
<xsl:for-each select="//Name[generate-id()=generate-id(key('names',text())[1])]">
<xsl:variable name="current" select="."/>
<xsl:apply-templates select="."/>
<xsl:apply-templates select="//Name[.=$current]/following-sibling::Year[1]"/>
<xsl:for-each select="//Name[.=$current]/following-sibling::Model[1]
[generate-id()=generate-id(key('models',text())[1])]">
<xsl:apply-templates select="."/>
</xsl:for-each>
</xsl:for-each>
</XYZ>
</xsl:template>
<xsl:template match="#*|node()">
<xsl:copy>
<xsl:apply-templates select="#*|node()"/>
</xsl:copy>
</xsl:template>
</xsl:transform>
Result:
<XYZ>
<Name>abc</Name>
<Year>1984</Year>
<Year>1987</Year>
<Model>qwe</Model>
<Name>qweqr</Name>
<Year>1977</Year>
<Model>tryet</Model>
</XYZ>
The duplicate nodes for Year and Model are removed as only unique values are selected in the xsl:for-each:
<xsl:for-each select="//Name[generate-id()=generate-id(key('names',text())[1])]">
The unique Name is copied, then all years that have a preceding Name with the value of the current unique Name:
<xsl:apply-templates select="//Name[.=$current]/following-sibling::Year[1]"/>
Then, the corresponding unique Model nodes are copied with a second xsl:for-each selecting only unique following Model nodes:
<xsl:for-each select="//Name[.=$current]/following-sibling::Model[1]
[generate-id()=generate-id(key('models',text())[1])]">
As there are already many answers on Stackoverflow for XSLT grouping using the Muenchian method, I just recommend the detailed explanation in this article by Jeni Tennison http://www.jenitennison.com/xslt/grouping/muenchian.xml.
As additional reference for XSLT grouping you can have a look at http://www.dpawson.co.uk/xsl/sect2/N4486.html
Update: As suggested as comment, this won't work in case of duplicate model values for different name values. Following adjusted XSLT
<?xml version="1.0" encoding="UTF-8" ?>
<xsl:transform xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
<xsl:output method="xml" omit-xml-declaration="yes" encoding="UTF-8 indent="yes" />
<xsl:strip-space elements="*"/>
<xsl:key name="names" match="Name" use="text()"/>
<xsl:template match="XYZ">
<XYZ>
<xsl:for-each select="//Name[generate-id()=generate-id(key('names',text())[1])]">
<xsl:variable name="current" select="."/>
<xsl:apply-templates select="."/>
<xsl:apply-templates select="//Name[.=$current]/following-sibling::Year[1]"/>
<xsl:for-each select="//Name[.=$current]/following-sibling::Model[1]
[not(.= preceding-sibling::Name[.=$current]
/preceding-sibling::Name[.=$current]
/following-sibling::Model)]">
<xsl:apply-templates select="."/>
</xsl:for-each>
</xsl:for-each>
</XYZ>
</xsl:template>
<xsl:template match="#*|node()">
<xsl:copy>
<xsl:apply-templates select="#*|node()"/>
</xsl:copy>
</xsl:template>
</xsl:transform>
works for the example XML provided in OP as well as the example XML suggested in the comment - only unique name values with optional multiple years and unique model values for a name, but the same model can also be listed for a different name.

Answer 2

I think you can use this stylesheet, which uses keys to select Year and Model(groups the Models too based on their Name and then with their value) based on their preceding Name(after grouping Names):
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
<xsl:output method="xml" indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:key name="Name" match="Name" use="."/>
<xsl:key name="Year" match="Year" use="preceding::Name[1]"/>
<xsl:key name="Model" match="Model" use="preceding::Name[1]"/>
<xsl:template match="XYZ">
<xsl:copy>
<xsl:for-each select="Name[count(. | key('Name', .)[1]) = 1]">
<xsl:copy-of select="."/>
<xsl:copy-of select="key('Year', .)"/>
<xsl:copy-of select="key('Model', .)[not(. = preceding::Model[preceding::Name[1] = current()])]"/>
</xsl:for-each>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>