def unify_equivalent_indels(df):
    """Unify equivalent indels with highest somatic probability

    Args:
        df (pandas.DataFrame): df after left-alignment
                               -> all equivalent indels have same 
                                  'chr', 'pos', 'ref', 'alt'
    Returns:
        df (pandas.Dataframe): de-duplicated dataframe
    """
    # to keep original order
    df["order"] = df.index

    # select one with highest somatic probability
    df = df.sort_values("prob_s", ascending=False)
    df = df.drop_duplicates(["chr", "pos", "ref", "alt"])
    df = df.sort_values("order")

    return df