title: Elasticsearch daily rolling index contains duplicate _id
body: <p>this maybe a silly question but I am using the daily rolling index to save my events with logstash, the config is simple as:</p>
  
  <pre><code>input: {..source..}
  filter: {..filter..}
  output: {
   elasticsearch: {
     document_id: %{my_own_guarantee_unique_id}
     index: myindex-%{+YYYY.MM.DD}
   }
  }
  </code></pre>
  
  <p>what I found was if there are events with same  <code>my_own_guarantee_unique_id</code> appears on different days, it will be created multiple times in these daily rolling indexes, ie. you can find event that has _id = 123 appearing in myindex-2015.06.21 and myindex-2015.06.22</p>
  
  <p>is this sort of duplicate out of box? what should I do to avoid it? any suggestion or readings will be appreciated, thanks!</p>