database to slugs (fun4)

import itertools
from collections import defaultdict
from string import ascii_lowercase as alphabet

flatten = itertools.chain.from_iterable
def double_generator(gen): return itertools.tee(gen, 2)


database = {
    0: {'name1': 'John', 'name2': 'Spam', 'born': '1985'},  # JohnSpam85a
    1: {'name1': 'John', 'name2': 'Spam', 'born': '1985'},  # JohnSpam85b
    2: {'name1': 'John', 'name2': 'Eggs', 'born': '1991'},  # JohnEggs91
    3: {'name1': 'John', 'name2': 'Eggs', 'born': '1992'},  # JohnEggs92
    4: {'name1': 'Emma', 'name2': 'Fish', 'born': '1995'},  # EmmaFish95a
    5: {'name1': 'Emma', 'name2': 'Fish', 'born': '1995'},  # EmmaFish95b
    6: {'name1': 'Mary', 'name2': 'Beer', 'born': '2000'},  # MaryBeer
    7: {'name1': 'Mary', 'name2': 'Wine', 'born': '2000'},  # MaryWine
    8: {'name1': 'Owen', 'name2': 'Wine', 'born': '2000'},  # Owen
    9: {'name1': 'Ruth', 'name2': 'Milk', 'born': '2000'}   # Ruth
}


def find_duplicates(number_to_slug, dup_gen_to_refine):
    slug_to_numbers = defaultdict(list)
    if dup_gen_to_refine is not None:
        for number in flatten(dup_gen_to_refine):
            slug = number_to_slug[number]
            slug_to_numbers[slug].append(number)
    else:
        for number, slug in number_to_slug.items():
            slug_to_numbers[slug].append(number)
    for block in slug_to_numbers.values():
        if len(block) > 1:
            yield block


def find_slugs(database):

    """
    Assign unique slugs to set of people. Slugs are as short as possible. (New elements only added to ensure uniqueness.)
    https://codereview.stackexchange.com/questions/285146
    """

    step_functions = [
        lambda _, n: database[n]['name1'],
        lambda _, n: database[n]['name2'],
        lambda _, n: database[n]['born'][2:4],
        lambda i, _: alphabet[i]
    ]

    number_to_slug = {n: '' for n in database}
    dup_gen_to_refine = None

    for step_fun in step_functions:
        dup_gen = find_duplicates(number_to_slug, dup_gen_to_refine)
        dup_gen_to_append, dup_gen_to_refine = double_generator(dup_gen)
        for block in dup_gen_to_append:
            for i, n in enumerate(block):
                number_to_slug[n] += step_fun(i, n)

    return number_to_slug