But I have a slightly different problem though.

<xsl:for-each select="root[not(foo/bar = preceding-sibling::foo/bar)]">
    <xsl:value-of select="."/><br/>
</xsl:for-each>

You probably want to select the foo instead of the root element:
 <xsl:for-each select="root/foo[not(bar = ../preceding-sibling::foo/bar)]">

but, it doesn't seem like working.

Well, there are many ways to "not work". You should be
more specific, for example "no output" or "I got this
output ..."