def unique_indexing(non_unique):
    """
    Convert a non_unique string pd.Series into a list of its indices of its unique list
    
    :type non_unique: pd.Series
    :param non_unique: containing non_unique values
    :return: list contains the index of non unique values indexed by the unique values
    """
    unique = non_unique.drop_duplicates().tolist()
    non_unique = non_unique.tolist()
    idxed_non_uniques = [unique.index(each_non_unique) for each_non_unique in non_unique]
    return idxed_non_uniques