DELETE FROM buchdurchschnittspreis2
  WHERE id in
    (SELECT buchdurchschnittspreis2.id FROM buchdurchschnittspreis2
      LEFT OUTER JOIN (select min(id) as id from buchdurchschnittspreis2
        GROUP BY (artnr, datum) as KeepRows
      ON buchdurchschnittspreis2.id=KeepRows.id
    WHERE KeepRows.id IS NULL);