code: def _candidate_merges(self, f):
          
          candidates = [self._get_feature(f.id)]
          c = self.conn.cursor()
          results = c.execute(
              constants._SELECT + '''
              JOIN duplicates ON
              duplicates.newid = features.id WHERE duplicates.idspecid = ?''',
              (f.id,)
          )
          for i in results:
              candidates.append(
                  feature.Feature(dialect=self.iterator.dialect, **i))
          return list(set(candidates))
docs: Identifies those features that originally had the same ID as `f`
          (according to the id_spec),  but were modified because of duplicate
          IDs.