Now I'm trying to identify the <column> nodes where duplicate @id
values exist. Where there are duplicates, I'd like to include the
nodes where the @file value is "doc." But the output should also
include the unique column nodes, all of which have a @file value of
"dev."

<xsl:stylesheet version="1.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output indent="yes"/>
  <xsl:key name="columns-by-id" match="column" use="@id"/>

  <xsl:template match="column[ @file = 'dev' ]">
    <xsl:if test="
      generate-id() =
      generate-id( key( 'columns-by-id', @id)[1])">
      <xsl:copy>
        <xsl:apply-templates select="@*|node()"/>
      </xsl:copy>
    </xsl:if>
  </xsl:template>

  <xsl:template match="@*|node()">
    <xsl:copy>
      <xsl:apply-templates select="@*|node()"/>
    </xsl:copy>
  </xsl:template>
</xsl:stylesheet>