Select first instance only with XPath

<root>
    <some_gunk/>
    <dupe_node>
        ...
        stuff I want
        ...
    </dupe_node>
    <bits_and_pieces/>
    <other_gunk/>
    <dupe_node>
        ...
        stuff I don't want
        ...
    </dupe_node>
    <more_gunk/>
</root>

/descendant::dupe_node[1]

<foo>
    <bar/>
    <foo>
        <bar/>
    </foo>
</foo>

//dupe_node[1]