def unique_cluster_indices(cluster_indx):
    """
    Return a unique list of cluster indices

    :param cluster_indx: Cluster index list of ClusterExpansionSetting
    """
    unique_indx = []
    for symmgroup in cluster_indx:
        for sizegroup in symmgroup:
            for cluster in sizegroup:
                if cluster is None:
                    continue
                for subcluster in cluster:
                    for indx in subcluster:
                        if indx not in unique_indx:
                            unique_indx.append(indx)
    return unique_indx