[xsl] Unique list of elements based in attribute

[Tech Guru <tech_gr@xxxxxxxxx>]

Hi Group,
            I am trying to generate stylesheet that would produce
elements with unique attribute value.

My xml:
<category_list>
    <category
id="16277" sort_order="10">
        <label language="en">Pianos and  keyboard
instruments</label>
        <category_list>
            <category id="16170"
sort_order="10">
                <label language="en">Grand pianos</label>
<category_list>
                    <category id="16323" sort_order="10">
<label language="en">Silent unit</label>
                     </category>
</category_list>
                </category>
    <category id="16377"
sort_order="10">

        <label language="en">Pianos and  keyboard
instruments</label>

        <category_list>

            <category id="16277"
sort_order="10">

                <label language="en">Grand pianos</label>
<category_list>

                    <category id="16323" sort_order="10">
<label language="en">Silent unit</label>

                     </category>
</category_list>

                </category>
        </category_list>
....
.....

Stylesheet:

<xsl:output method="xml" version="1.0" encoding="UTF-8"
indent="yes"/>
    <xsl:template match="/">
        <Category_List>
<xsl:apply-templates select="category_list"/>
        </Category_List>
</xsl:template>
    <!-- Any occurrence of category element in current context
is applied -->
    <xsl:template match="category">
        <!-- check whether,
category with same id is already processed or not -->
        <!-- Here I am
trying to find any sibling categories with same id or not.-->
        <xsl:if
test ="not($id=preceding-sibling::category/@id)">
        
        <!--xsl:if
test="@id!=ancestor::category/@id"-->
            <Category
xmlns:YMIA="urn:schemas-music-yamaha-com:ymia">
                <xsl:attribute
name="Id">
                    <xsl:value-of select="@id"/>
</xsl:attribute>
            </Category>
        </xsl:if>
        <!--
Selects category list of current category. XSLT logic no worry :)  -->
<xsl:apply-templates select="category_list"/>
    </xsl:template>

<xsl:if
test ="not($id=preceding-sibling::category/@id)"> : This does not seem to
work. Any input.

-R
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