Merging Solr index

[Norskog, Lance]

Hi-
 
http://wiki.apache.org/solr/MergingSolrIndexes recommends using the
Lucene contributed app IndexMergeTool to merge two Solr indexes. What
happens if both indexes have records with the same unique key? Will they
both go into the new index?
 
Is the implementation of unique IDs in the Solr java or in Lucene? If it
is in Solr, how would I hackup a Solr IndexMergeTool?
 
Cheers,
 
Lance Norskog