find_duplicates.py

#!/usr/bin/env python3
import sys

def find_duplicates(data_generator):
    """Find duplicates in the given data.

    The `data_generator` is an iterable over strings, so it can be
    iterated for example using a `for` cycle:

      for item in data_generator: ...

    It can be iterated multiple times.

    The goal is to return a list of duplicated entries, reporting each duplicated
    entry only once.
    """

    raise NotImplementedError()