Do something only on distinct XML nodes

I'm having hard times in triggering an xsl action only on distinct values. Here's an excerpt of my XML file:

<?xml version="1.0" encoding="UTF-8" ?> 
<Doc Data="February 2010">
  <Title>Title 1</Title> 
  <Subject>Subject 1</Subject> 
</Doc>
<Doc Data="January 2010">
  <Title>Title 2</Title> 
  <Subject>Subject 2</Subject> 
</Doc>
<Doc Data="January 2010">
  <Title>Title 3</Title> 
  <Subject>Subject 3</Subject> 
</Doc>

And this is my xsl file:

<?xml version="1.0" encoding="iso-8859-1"?>
<xsl:transform  xmlns:xsl="http://www.w3.org/1999/XSL/Transform" 
                xmlns:msxsl="urn:schemas-microsoft-com:xslt" 
                xmlns:objUtility="urn:utility"
                version="1.0">
<xsl:output method="html" indent=开发者_运维百科"no" encoding="ISO-8859-1"/>

  <xsl:template match="Doc">
    <xsl:if test="not(@Data = preceding-sibling::Doc/@Data)">
      <xsl:value-of select="@Data" />
    </xsl:if>
  </xsl:template>

<xsl:apply-templates />

The problem is that, with this code, the preceding-sibling::Doc/@Data element always contains ONLY THE FIRST element, so basically I get a distinct value only for February and not for January. Where am I wrong? I must use XSL 1.0

If you wish to ask “Is there any preceding sibling of type Doc with a Data attribute equal to the Data attribute of the current node?”, the XPath for that should look like

<xsl:if test="not(preceding-sibling::Doc[@Data = current()/@Data])">