Duplicate results from query trying to find cross-indexed multiple tables
Never mind. I re-wrote it and this seems to work;

SET search_path="SCHEMA_NAME";
SELECT e1.label, c1.label, h1.contentsid, h1.group
FROM hostdata as h1
INNER JOIN (
    SELECT contentsid, group
    FROM hostdata 
        GROUP BY contentsid, group
    HAVING COUNT(id) &gt; 1
    ) dup 
    ON h1.group = dup.group AND h1.contentsid = dup.contentsid
INNER JOIN class as c1
  on h1.classid = c1.id
INNER JOIN element as e1
  on h1.elementid = e1.id
ORDER BY t1.group, t1.contentsid;