title: MongoDB: how to insert document without repeat
body: <p>Suppose I already have some data in database:</p>
  
  <pre><code>{
     id: 001,
     title: "title_1"
  }
  {
     id: 002,
     title: "title_2"
  }
  {
     id: 003,
     title: "title_3"
  }
  </code></pre>
  
  <p>then I want to insert some new documents:</p>
  
  <pre><code>{
     id: 003    // id repeat
     title: "title_4"
  }
  {
     id: 004
     title: "title_5"
  }
  </code></pre>
  
  <p>but I don't want to insert the <code>{ id:003 }</code> item, because some guy in the database already has the same id.</p>
  
  <p>So, how can I let the MongoDB ignore the repeat value(with specified key) item when I insert new data?</p>