remove duplicate

I have follow xml-data.

<result>
<row>
<cell>Paris</cell>
<cell>110</cell>
</row>
<row>
<cell>Paris</cell>
<cell>120</cell>
</row>
<row>
<cell>Paris</cell>
<cell>90</cell>
</row>
<row>
<cell>London</cell>
<cell>200</cell>
</row>
<row>
<cell>London</cell>
<cell>80</cell>
</row>
</result>

If i use XPath in an XSL-sheet, how can i get a list of all cities.
An expression like //cell[position()=1] does not work. because it will return Paris three times and London twice.

Is there a operator which does the same as the DISTINCT keyword in SQL, to remove all duplicates?
Or how can i use XQuery?

Thanks in advance

Bye
Peter