In an XForms select1 itemset, ensure that the nodeset only contains unique values

select-box unique.html

      <xf:select1 ref="instance('policeforce')/selectedforce" selection="closed" >
         <xf:itemset nodeset="instance('report-transformed')/body//tr[not(td[3] = preceding-sibling::tr/td[3])]">
            <xf:label ref="td[3]" />
            <xf:value ref="td[3]" />
         </xf:itemset>
      </xf:select1>