def get_user_names(L):
    """
    for each line in twitter data
        slice out username
        if its not in username list add it
    :param L - list of twitter data:
    :return user_names - list of user names:
    """

    user_names = list()

    for item in L:

        user_name = item[0]

        if user_name not in user_names:
            user_names.append(user_name)

    user_names = sorted(user_names)

    return user_names