'Combines the series from multiple lists, making sure there are no doubles.
If keyword argument log_once is set to True, an error message will be printed if a series
is listed more than once, otherwise log_once will be used.'

def combine_series_lists(self, *series_lists, **kwargs):

    unique_series = {}
    for series_list in series_lists:
        for series in series_list:
            (series, series_settings) = list(series.items())[0]
            if (series not in unique_series):
                unique_series[series] = series_settings
            else:
                if kwargs.get(u'log_once'):
                    log_once((u'Series    `%s`    is    already    configured    in    series    plugin' % series), log)
                else:
                    log.warning(u'Series    `%s`    is    configured    multiple    times    in    series    plugin.', series)
                unique_series[series].update(series_settings)
    return [{s: unique_series[s]} for s in sorted(unique_series, reverse=True)]
