If you have many documents with the same value of the same field, this source code can help you. Using terms aggregator and top hits aggregator is required.

My index :

• Doc 1 {domain: ‘domain1.fr’, name: ‘name1′, date: ’01-01-2014’}
• Doc 2 {domain: ‘domain1.fr’, name: ‘name1′, date: ’01-02-2014’}
• Doc 3 {domain: ‘domain2.fr’, name: ‘name2′, date: ’01-03-2014’}
• Doc 4 {domain: ‘domain2.fr’, name: ‘name2′, date: ’01-04-2014’}
• Doc 5 {domain: ‘domain3.fr’, name: ‘name3′, date: ’01-05-2014’}
• Doc 6 {domain: ‘domain3.fr’, name: ‘name3′, date: ’01-06-2014’}

My result (deduplication result by domain field) :

• Doc 6 {domain: ‘domain3.fr’, name: ‘name3′, date: ’01-06-2014’}
• Doc 4 {domain: ‘domain2.fr’, name: ‘name2′, date: ’01-04-2014’}
• Doc 2 {domain: ‘domain1.fr’, name: ‘name1′, date: ’01-02-2014’}

Elasticsearch query :

/POST http://localhost:9200/test/dedup/_search?search_type=count&pretty=true
"aggs":{
    "dedup" : {
      "terms":{
        "field": "domain"
       },
       "aggs":{
         "dedup_docs":{
           "top_hits":{
             "size":1
           }
         }
      }
    }
  }
}

I originally asked a question here.