MoreLikeThis gone wrong

import java.io.IOException;

import org.apache.lucene.analysis.standard.StandardAnalyzer;
import org.apache.lucene.document.Document;
import org.apache.lucene.document.Field;
import org.apache.lucene.document.Field.Index;
import org.apache.lucene.document.Field.Store;
import org.apache.lucene.index.CorruptIndexException;
import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.IndexWriter;
import org.apache.lucene.index.IndexWriterConfig;
import org.apache.lucene.search.IndexSearcher;
import org.apache.lucene.search.Query;
import org.apache.lucene.search.ScoreDoc;
import org.apache.lucene.search.TopDocs;
import org.apache.lucene.search.similar.MoreLikeThis;
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.LockObtainFailedException;
import org.apache.lucene.store.RAMDirectory;
import org.apache.lucene.util.Version;


public class Main {
    public static void main(String[] args) throws CorruptIndexException, LockObtainFailedException, IOException{
        //Setting up the index
        StandardAnalyzer analyzer = new StandardAnalyzer(Version.LUCENE_36);
        Directory index = new RAMDirectory();
        IndexWriterConfig config = new IndexWriterConfig(Version.LUCENE_36, analyzer);

        //Populate the index
        IndexWriter w = new IndexWriter(index, config);
        addBook(w, "My first Bratwurst", "'Amazing' - Financial Times");
        addBook(w, "My first Bratwurst", "'Amazing' - Financial Times"); //identical to the first one
        addBook(w, "Financial Studies - Part 1", "Rainer Bratwurst with his first book");
        addBook(w, "Financial Studies - Part 2", "Author Rainer Bratwurst does not so well in this one");
        addBook(w, "Financial Studies - Part 3", "Rainer Bratwurst my favorite author");
        addBook(w, "Amazing Times", "Author Rainer Bratwurst strikes again. First choice for my Kids.");
        addBook(w, "Amazing Times - Back again", "This one is not like part 1 from author Rainer Bratwurst");
        addBook(w, "Why Times New Roman is Amazing", "An essay I wrote in my first year at college");
        w.close();

        //Setting up MoreLikeThis
        IndexReader ir = IndexReader.open(index);
        MoreLikeThis mlt = new MoreLikeThis(ir);
        mlt.setFieldNames(new String[]{"title", "content"});
        mlt.setMinTermFreq(1);
        mlt.setMinDocFreq(1);

        //Build the MLT-Query for the first document in the index and execute it
        Query q = mlt.like(0);
        IndexSearcher is = new IndexSearcher(ir);
        TopDocs result = is.search(q, 10);
        for(ScoreDoc sd : result.scoreDocs){
            //Print all titles similar to "My first Bratwurst" - the duplicate is not found
            Document document = ir.document(sd.doc);
            String title = document.get("title");
            System.out.println(title);
        }

    }
    private static void addBook(IndexWriter w, String title, String content) throws IOException {
          Document doc = new Document();
          doc.add(new Field("title", title, Store.YES, Index.ANALYZED));
          doc.add(new Field("content", content, Store.YES, Index.ANALYZED));
          w.addDocument(doc);
        }

}