Elastica add documents overrides existing ones

When i try to add new documents to an index type , i loose existing documents which are overwritten by the new added ones . The problem can be related to the id of each added document ??

Here is the code :

     $elasticaClient = new \Elastica\Client(array(
                'host' => $this->container->getParameter('elastic_host'),
                'port' => $this->container->getParameter('elastic_port')
            ));
     $elasticaIndex = $elasticaClient->getIndex('app');
           $elasticaIndex->create(
            array(
                'number_of_shards' => 4,
                'number_of_replicas' => 1,
                'analysis' => array(
                    'analyzer' => array(
                        'indexAnalyzer' => array(
                            'type' => 'custom',
                            'tokenizer' => 'standard',
                            'filter' => array('lowercase', 'mySnowball')
                        ),
                        'searchAnalyzer' => array(
                            'type' => 'custom',
                            'tokenizer' => 'standard',
                            'filter' => array('standard', 'lowercase',    'mySnowball')
                        )
                    ),
                    'filter' => array(
                        'mySnowball' => array(
                            'type' => 'snowball',
                            'language' => 'German'
                        )
                    )
                )
            ),
            true
        );
     $elasticaType = $elasticaIndex->getType('type');
      $mapping = new \Elastica\Type\Mapping();
      $mapping->setType($elasticaType);
      $mapping->setParam('index_analyzer', 'indexAnalyzer');
      $mapping->setParam('search_analyzer', 'searchAnalyzer');
      $mapping->setProperties(array(
            'id'      => array('type' => 'string'),
            'title'     => array('type' => 'string'),
            'duration'     => array('type' => 'string'),
            'start'     => array('type' => 'string'),
            'end'     => array('type' => 'string'),
        ));

        // Send mapping to type
        $mapping->send();

    $documents = array(); 
            foreach($medias as $media) { 
                $id = uniqid() ;
                $documents[] = new \Elastica\Document(
                    $id,
                    array(
                    'id'       => $id,
                    'title'    => $media['title'],
                    'duration' => $media['duration'],
                    'start'    => $media['start'],
                    'end'      => $media['end'],

                    )
                );
            }

 $elasticaType->addDocuments($documents);
 $elasticaType->getIndex()->refresh();

Please i need your help . Thank you

PHP does not recommend using uniqid for this use case. Since you are wanting a random, safe id, let Elasticsearch do it for you. The Elastica Document construct method notes that the id field is optional. So don't pass it and let Elasticsearch issue the id.