def METHOD_NAME(sample_features, sample_importance_df_1, sample_importance_df_2):
    # test the scenario where previous feature importance df exists and its importance estimates come from both same and different fitted models
    prev_df, curr_df = sample_importance_df_1, sample_importance_df_2
    using_prev_fit_fi = set([sample_features[0]])
    result_df = merge_importance_dfs(prev_df, curr_df, using_prev_fit_fi=using_prev_fit_fi).sort_index()
    assert [score if score == score else None for score in result_df["importance"].tolist()] == [-0.1, 0.0, 0.1, 1.0, None]
    assert result_df["n"].tolist() == [5, 15, 10, 5, 0]
    assert using_prev_fit_fi == set()