def idxs_of_duplicates(lst):
    """ Returns the indices of duplicate values.
    """
    idxs_of = dict({})
    dup_idxs = []
    for idx, value in enumerate(lst):
        idxs_of.setdefault(value, []).append(idx)
    for idxs in idxs_of.values():
        if len(idxs) > 1:
            dup_idxs.extend(idxs)
    return dup_idxs