I don't know where you read that - the documentation is pretty clear that you should not merge indexes with duplicate values for the unique key as it will result in duplicates in the resulting index:
https://wiki.apache.org/solr/MergingSolrIndexes
https://cwiki.apache.org/confluence/display/solr/Merging+Indexes
http://lucene.472066.n3.nabble.com/Duplicate-docs-when-merging-indices-td1262043.html