How do I prevent duplicate entries into a list, and then ideally, sort that list? What I'm doing, is when information at one level is missing, taking the information from a level below it, to building the missing list, in the level above. Currently, I have XML similar to this:

<c03 id="ref6488" level="file">
    <did>
        <unittitle>Clinic Building</unittitle>
        <unitdate era="ce" calendar="gregorian">1947</unitdate>
    </did>
    <c04 id="ref34582" level="file">
        <did>
            <container label="Box" type="Box">156</container>
            <container label="Folder" type="Folder">3</container>
        </did>
    </c04>
    <c04 id="ref6540" level="file">
        <did>
            <container label="Box" type="Box">156</container>
            <unittitle>Contact prints</unittitle>
        </did>
    </c04>
    <c04 id="ref6606" level="file">
        <did>
            <container label="Box" type="Box">154</container>
            <unittitle>Negatives</unittitle>
        </did>
    </c04>
</c03>

I then apply the following XSL:

<xsl:template match="c03/did">
    <xsl:choose>
        <xsl:when test="not(container)">
            <did>
                <!-- If no c03 container item is found, look in the c04 level for one -->
                <xsl:if test="../c04/did/container">

                    <!-- If a c04 container item is found, use the info to build a c03 version -->
                    <!-- Skip c03 container item, if still no c04 items found -->
                    <container label="Box" type="Box">

                        <!-- Build container list -->
                        <!-- Test for more than one item, and if so, list them, -->
                        <!-- separated by commas and a space -->
                        <xsl:for-each select="../c04/did">
                            <xsl:if test="position() &gt; 1">, </xsl:if>
                            <xsl:value-of select="container"/>
                        </xsl:for-each>
                    </container>                    
            </did>
        </xsl:when>

        <!-- If there is a c03 container item(s), list it normally -->
        <xsl:otherwise>
            <xsl:copy-of select="."/>
        </xsl:otherwise>
    </xsl:choose>
</xsl:template>

But I'm getting the "container" result of

<container label="Box" type="Box">156, 156, 154</container>

when what I want is

<container label="Box" type="Box">154, 156</container>

Below is the full result that I'm trying to get:

<c03 id="ref6488" level="file">
    <did>
        <container label="Box" type="Box">154, 156</container>
        <unittitle>Clinic Building</unittitle>
        <unitdate era="ce" calendar="gregorian">1947</unitdate>
    </did>
    <c04 id="ref34582" level="file">
        <did>
            <container label="Box" type="Box">156</container>
            <container label="Folder" type="Folder">3</container>
        </did>
    </c04>
    <c04 id="ref6540" level="file">
        <did>
            <container label="Box" type="Box">156</container>
            <unittitle>Contact prints</unittitle>
        </did>
    </c04>
    <c04 id="ref6606" level="file">
        <did>
            <container label="Box" type="Box">154</container>
            <unittitle>Negatives</unittitle>
        </did>
    </c04>
</c03>

Thanks in advance for any help!