code: HashSet<String> checkDupColumnNames() {
      HashSet<String> conflictingNames = new HashSet<>();
      if( null==_column_names ) return conflictingNames;
      HashSet<String> uniqueNames = new HashSet<>();
      for( String n : _column_names)
        if( !uniqueNames.add(n) ) conflictingNames.add(n);
      return conflictingNames;
    }
docs: Set of duplicated column names