To make this (and other things) simpler, I added @xml:id attributes to the root elements of all our files. For the record, to save a couple of minutes, here's the code to get duplicate ids for pasting into a spreadsheet. Don't forget to set the number of results high enough.

declare default element namespace "http://www.tei-c.org/ns/1.0";

for $id in distinct-values(//entry/@xml:id)
let $c := count(//entry[@xml:id=$id]),
$docs := if ($c gt 1) then //TEI[descendant::entry[@xml:id=$id]]/@xml:id else ()
where $c gt 1
order by $id
return concat($id, '	', $c, '	', string-join($docs, ', '))