def remove_duplicate_columns(x_train, x_test=None):
    """
    Removes columns whose values are exact duplicates of each other.
    
    Parameters
    ----------
    x_train: Dataframe or array like - 2d
        Dataset.

    x_test: Dataframe or array like - 2d
        Testing dataset, by default None.
    
    Returns
    -------
    Dataframe, *Dataframe
        Transformed dataframe with rows with a missing values in a specific row are missing

    Returns 2 Dataframes if x_test is provided.
    """
    x_train = x_train.T.drop_duplicates().T
    if x_test is not None:
        x_test = x_test.T.drop_duplicates().T
    return x_train, x_test
