Affichage des numéros de ligne en double

utilisez GROUP_CONCAT

SELECT  a.data, 
        a.rowNumber, 
        GROUP_CONCAT(b.rowNumber)
FROM    DuplicateRows AS a
        INNER JOIN DuplicateRows AS b 
            ON  a.data = b.dataAND a.Id != b.Id
GROUP BY a.data, a.rowNumber

Démo SQLFiddle