I tried it with the example json documents, and even if I add overwrite=false to the URL, it still overwrites.
Do this twice: curl 'http://localhost:8983/solr/update/json?commit=true&overwrite=false' --data-binary @books.json -H 'Content-type:application/json' Then do this query: curl 'http://localhost:8983/solr/select?q=title:monsters&wt=json&indent=true' --Dave