Solr Result Grouping / Field Collapsing Improvements by Yonik Seeley
From the post:
I previously introduced Solr’s Result Grouping, also called Field Collapsing, that limits the number of documents shown for each “group”, normally defined as the unique values in a field or function query.
Since then, there have been a number of bug fixes, performance improvements, and feature enhancements. You’ll need a recent nightly build of Solr 4.0-dev, or the newly released LucidWorks Enterprise v1.6, our commercial version of Solr.
A short but useful article on new grouping capabilities in Solr.
What you do with results once they are grouped, which could include “merging,” is up to you.