title: Elasticsearch/Logstash duplicating output when use schedule
body: <p>Im using Elasticsearch with Logstash. 
  I want to update indexes when database changes. So i decided to use LS <strong>schedule</strong>. But every 1 minute output appended by database table records. 
  Example: contract table has 2 rows.
  First 1 minute total: 2, 1 minute after total output is : 4;
  How can i solve this?</p>
  
  <p>There is my config file. Command is <em>bin/logstash -f contract.conf</em></p>
  
  <pre><code>input {
          jdbc {
              jdbc_connection_string =&gt; "jdbc:postgresql://localhost:5432/resource"
              jdbc_user =&gt; "postgres"
              jdbc_validate_connection =&gt; true
              jdbc_driver_library =&gt; "/var/www/html/iltodgeree/logstash/postgres.jar"
              jdbc_driver_class =&gt; "org.postgresql.Driver"
              statement =&gt; "SELECT * FROM contracts;"
              schedule =&gt; "* * * * *"
              codec =&gt; "json"
          }
      }
  
      output {
          elasticsearch {
              index =&gt; "resource_contracts"
              document_type =&gt; "metadata"
              hosts =&gt; "localhost:9200"
          }
      }
  </code></pre>