def common_element(my_dict):
    my_dict = {
        "A": "K",
        "B": "D",
        "C": "A",
        "D": "Z"
    }

    empty = []
    empty_key = []
    empty_value = []
    for key, value in my_dict.items():
        print(key)
        empty_key.append(key)
        empty_value.append(value)

        # for keys in empty_key:
        if key in empty_value:
            empty.append(key)

        elif value in empty_key:
            empty.append(value)

    return empty


print(common_element(my_dict))