How to use XSLT to create distinct values

I have XML like this:

<items>
  <item>
    <products>
      <product>laptop</product>
      <product>charger</product>
    </products>
  </item>
  <item>
    <products>
      <product>laptop</product>
      <product>headphones</product>  
    </products>  
  </item>
</items>

I want it to output like

laptop
charger
headphones

I was trying to use distinct-values() but I guess i m doing something wrong. Can anyone tell me how to achieve this using distinct-values()? Thanks.

<xsl:template match="/">            
  <xsl:for-each select="//products/product/text()">
    <li>
      <xsl:value-of select="distinct-values(.)"/>
    </li>               
  </xsl:for-each>
</xsl:template>

but its giving me output like this:

<li>laptop</li>
<li>charger</li>
<li>laptop></li>
<li>headphones</li>