title: E11000 duplicate key error on multiple unique keys in mongoDB
body: <p>I have 2 unique keys in my userSchema: </p>
  
  <pre><code>email: {
          type: String,
          required: true,
          unique: true
      },
  username: {
          type: String,
          required: true,
          unique: true
      }
  </code></pre>
  
  <p>You can't add neither username nor email which already exist in the database. 
  How can I tell user which one exactly already exists in the database, because there is only one error code?</p>
  
  <pre><code>if (err.code == 11000)
      return res.status(422).send(['Duplicate email adrress found.']);
  </code></pre>