Getting unique items from elements

I have the following snippet of XML:

<books>
  <book>
    <category>Children</category>
    <category>English</category>
    <title>Mary Poppins</title>
  </book>
  <book>
    <category>Children</category>
    <category>Spanish</category>
    <title>Los Perdidos</title>
  </book>
</books>

Which I currently (and erroneously) transform using:

  <xsl:template match="/">
      <xsl:for-each select="books/book/category">
        <xsl:value-of select="."/>,
      </xsl:for-each>
  </xsl:template>

The output as you would expect includes the category of "Children" twice. Is
there a way in XSL to extract only the unique elements from <category>?

Thank you!