Exercise 5: Duplicate Letters — the Two-List Way — Possible Solution
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word = "bookkeeper"

my_list = []
duplicates = []

for ch in word:
    if ch not in my_list:
        my_list.append(ch)
    elif ch not in duplicates:
        duplicates.append(ch)

print(duplicates)

Output:
['o', 'k', 'e']

WHY THIS WORKS AS AN ANSWER
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"bookkeeper" is a genuinely good stress test for the elif guard: "o"
appears twice, "k" appears twice, and "e" appears three times in a
row (the "eee" in "keeper"). Walking through it — b, o (new), o
(seen, not yet flagged -> added to duplicates), k (new), k (seen,
not yet flagged -> added to duplicates), e (new), e (seen, not yet
flagged -> added to duplicates), p, e (seen, ALREADY flagged this
time -> elif ch not in duplicates is False, so nothing happens), r.

That last e is exactly why the elif ch not in duplicates check
exists — without it, "e" would have been appended to duplicates
twice, once for the second occurrence and again for the third,
breaking the "print each duplicate exactly once" requirement.