private Result blendResultsUniquely(Result result, Query q, int offset,
                                        int hits, List<HitGroup> groups, Execution execution) {
        DocumentMerger m = new UniqueMerger(result, groups.get(0), new HashSet<>(20));

        m.scanResult(execution);
        m.mergeResults(groups, execution);
        return sortAndTrimResults(m.getResult(), q, offset, hits, execution);
    }
Produce a single blended hit list from a group of hitgroups.

This assumes that all hits are organized into hitgroups. If not, blending will not be performed.
container-search/src/main/java/com/yahoo/prelude/searcher/BlendingSearcher.java