finding the first unique tagName

Starting with this snip:
        <tag id="Quoted_Text">
                        <usage>
                                <para>
                                        <tagName idref="Quoted_Text"/> 
identifies text that is a direct quotation usually from the Old 
Testament. <tagName idref="Quoted_Text"/> can be found in prose or poetry. You 
only need to mark your text with <tagName idref="Quoted_Text"/> if you want a 
special look for the text in focus or you want to search 
for all <tagName idref="Quoted_Text"/>.</para>
                                <note>
                                        <para>
                                                <tagName 
idref="Citation_Line1"/> and <tagName idref="Citation_Line2"/> is a one tag 
method of identifying the paragraphs that are quoted from 
another location, like the Old Testament.</para>
                                </note>
                        </usage>
                </tag>

I need to be able to identify the first occurrence of each unique 
tagName/@idref 

so I am looking at something like:
<when test="the current tagName/@idref is the first tagName/@idref in 
usage">

ancestor::usage/descendant::tagName/@idref 
gives me all the tagName/@idref 
Quoted_Text 
Quoted_Text 
Quoted_Text 
Citation_Line1 
Citation_Line2

but I want to identify

Quoted_Text ***
Quoted_Text 
Quoted_Text 
Citation_Line1 *** 
Citation_Line2 ***

just these three

Thanks for your help.

I am using XSLT 2.0.

Jim Albright
704 843-0582
Wycliffe Bible Translators



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