code: public static List<Writable> getUniqueSequence(String columnName, Schema schema,
                      JavaRDD<List<List<Writable>>> sequenceData) {
          JavaRDD<List<Writable>> flattenedSequence = sequenceData.flatMap(new SequenceFlatMapFunction());
          return getUnique(columnName, schema, flattenedSequence);
      }
docs: Get a list of unique values from the specified column of a sequence
  
  @param columnName      Name of the column to get unique values from
  @param schema          Data schema
  @param sequenceData    Sequence data to get unique values from
  @return