immediately preceding:: / following::

I have my <NodeID> elements already sorted.
I am testing whether or not there are duplicate NodeIDs.
If there are, I need to only include one of them.
preceding:: or following:: would work.
This works fine for small files, but the order is N^N.
Could someone please give me some performance tips?

I am using the following lines:
     <xsl:if test = "NodeID[not(.=preceding::NodeID)]">
     ...
     </xsl:if>

Thanks in advance,
     Matt


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