xml - How to select unique nodes

Question

I found this page describing the Muenchian method, but I think I'm applying it wrong.

Consider that this would return a set of ages:

/doc/class/person/descriptive[(@name='age')]/value

1..2..2..2..3..3..4..7

But I would like a nodeset only one node for each age.

1..2..3..4..7

Each of these seem to return all of the values, instead of unique values:

/doc/class/person/descriptive[(@name='age')][not(value=preceding-sibling::value)]/value
/doc/class/person/descriptive[(@name='age')]/value[not(value=preceding-sibling::value)]

What am I missing?

Answer 1

Here's an example:

<root>
    <item type='test'>A</item>
    <item type='test'>B</item>
    <item type='test'>C</item>
    <item type='test'>A</item>
    <item type='other'>A</item>
    <item type='test'>B</item>
    <item type='other'>D</item>
    <item type=''>A</item>
</root>

And the XPath:

//preceding::item/preceding::item[not(.=preceding-sibling::item)]/text()

Results: A B C D

EDIT: As mousio commented this doesn't capture the last item in a list if it's the only time it appears. Taking that and F?anor's comment into account, here's a better solution:

/root/item[not(.=preceding-sibling::item)]