def duplicate(list_a: list):
    """Problem 14: Duplicate the Elements of a list.

    Parameters
    ----------
    list_a : list
        The input list

    Returns
    -------
    list
        A list of duplicated elements in order

    Raises
    ------
    TypeError
        If the given argument is not of `list` type

    """
    if not isinstance(list_a, list):
        raise TypeError('The argument given is not of `list` type.')

    duplicated = []
    for x in list_a:
        duplicated.extend([x, x])

    return duplicated