from typing import List


def repeats(L: List[int]) -> List[int]:
    """Returns a sorted list of unique elements that are present in L more than once."""
    return list(sorted({x for x in L if L.count(x) > 1}))


def testRepeats():
    print("Testing repeats()...", end="")
    assert repeats([1, 2, 3, 2, 1]) == [1, 2]
    assert repeats([1, 2, 3, 2, 2, 4]) == [2]
    assert repeats([1, 5, 3, 5, 2, 3, 2, 1]) == [1, 2, 3, 5]
    assert repeats([7, 9, 1, 3, 7, 1]) == [1, 7]
    assert repeats(list(range(100))) == []
    assert repeats(list(range(100)) * 5) == list(range(100))

    # Verify that the function is nonmutating:
    L = [1, 2, 3, 2, 1]
    repeats(L)
    assert L == [1, 2, 3, 2, 1]
    print("Passed!")


def main():
    testRepeats()


main()