Applying second grouping for all single emenets in a group - xslt-2.0

Question

I used for-each-group, if want to group items based on key and then apply second grouping if they are not part of any group.
My sample xml is
<items>
<item id="123" name="Java">
<price></price>
<description></description>
</item>
<item id="123" name="Java and XML">
<price></price>
<description></description>
</item>
<item id="234" name="python">
<price></price>
<description></description>
</item>
<item id="456" name="scala">
<price></price>
<description></description>
</item>
<item id="" name="python">
<price></price>
<description></description>
</item>
<item id="768" name="scala">
<price></price>
<description></description>
</item>
<item id="891" name="angular">
<price></price>
<description></description>
</item>
</items>
First i want to group by id and if there are multiple elements in group then i will form a group, otherwise i will apply another grouping with name then form the group, finally if it is not any grouping then make its own group.
Output should be some thing like this
<items>
<group>
<item id="123" name="Java">
<price></price>
<description></description>
</item>
<item id="123" name="Java and XML">
<price></price>
<description></description>
</item>
</group>
<group>
<item id="234" name="python">
<price></price>
<description></description>
</item>
<item id="" name="python">
<price></price>
<description></description>
</item>
</group>
<group>
<item id="456" name="scala">
<price></price>
<description></description>
</item>
<item id="768" name="scala">
<price></price>
<description></description>
</item>
</group>
<group>
<item id="891" name="angular">
<price></price>
<description></description>
</item>
</group>
</items>
How to apply grouping for the ones which are not in any group ? is for-each-group the right one for this.
Update
I tried this approach
<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output omit-xml-declaration="yes" indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:variable name="nonMatched" as="element()*">
<xsl:for-each-group select="./items/item" group-by="#id">
<xsl:if test="count(current-group()) lt 2">
<xsl:sequence select="current-group()"/>
</xsl:if>
</xsl:for-each-group>
</xsl:variable>
<xsl:template match="/">
<items>
<xsl:for-each-group select="/items/item" group-by="#id">
<xsl:if test="count(current-group()) gt 1">
<group>
<xsl:copy-of select="current-group()"/>
</group>
</xsl:if>
</xsl:for-each-group>
<xsl:for-each-group select="$nonMatched" group-by="#name">
<group>
<xsl:copy-of select="current-group()"/>
</group>
</xsl:for-each-group>
</items>
</xsl:template>

Answer 1

One way would be to push the items through different templates where you use keys to identify the groups and the conditions you have for the groups and then form the group for the first item in each group and use an empty template for the other items in each group; note that the following exploits the priority imposed by order https://www.w3.org/TR/xslt-30/#conflict so the order used below for the templates is important, although you could as well use priority attributes to impose your rules to favour #id based grouping, then #name based grouping, then group any item (i.e. match="item") not covered by the other grouping conditions:
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
exclude-result-prefixes="#all"
version="3.0">
<xsl:mode on-no-match="shallow-copy"/>
<xsl:output method="xml" indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:key name="id" match="item[normalize-space(#id)]" use="#id"/>
<xsl:key name="name" match="item" use="#name"/>
<xsl:template match="item">
<group>
<xsl:copy-of select="."/>
</group>
</xsl:template>
<xsl:template match="item[key('name', #name)[2] and . is key('name', #name)[1]]">
<group>
<xsl:copy-of select="key('name', #name)"/>
</group>
</xsl:template>
<xsl:template match="item[key('name', #name)[2] and not(. is key('name', #name)[1])]"/>
<xsl:template match="item[key('id', #id)[2] and . is key('id', #id)[1]]">
<group>
<xsl:copy-of select="key('id', #id)"/>
</group>
</xsl:template>
<xsl:template match="item[key('id', #id)[2] and not(. is key('id', #id)[1])]"/>
</xsl:stylesheet>
Online at https://xsltfiddle.liberty-development.net/bdxtqt, uses XSLT 3 but you could replace the xsl:mode declaration used at the beginning with the identity template
<xsl:template match="#* | node()">
<xsl:copy>
<xsl:apply-templates select="#* | node()"/>
</xsl:copy>
</xsl:template>
to have it work with an XSLT 2 processor. Only caveat there is an XSLT 2 processor could resort to reporting an error if there are multiple matches instead of taking the last matching template, I don't remember which XSLT 2 processor does that, but as said explicitly using priorities would solve that.
Your posted approach should also work:
<xsl:template match="items">
<xsl:copy>
<xsl:variable name="nonMatched" as="element()*">
<xsl:for-each-group select="item" group-by="#id">
<xsl:sequence
select="if (not(current-group()[2]))
then .
else ()"/>
</xsl:for-each-group>
</xsl:variable>
<xsl:for-each-group select="item except $nonMatched" group-by="#id">
<group>
<xsl:apply-templates select="current-group()"/>
</group>
</xsl:for-each-group>
<xsl:for-each-group select="$nonMatched" group-by="#name">
<group>
<xsl:apply-templates select="current-group()"/>
</group>
</xsl:for-each-group>
</xsl:copy>
</xsl:template>
https://xsltfiddle.liberty-development.net/3NzcBtv