def _drop_duplicate_labels(a_labels, b_labels):
    """Combine lists `a_labels` and `b_labels` into a new list, dropping any duplicates."""
    a_labels = list(a_labels)
    b_labels = list(b_labels)
    for i, lbl in enumerate(a_labels):
        if lbl in b_labels:
            j = b_labels.index(lbl)
            a_labels[i] = None
            b_labels[j] = None
    a_labels.extend(b_labels)
    return a_labels
