title: Set custom immutablejs keys to prevent data duplication?
body: <p>I have data that comes in the following format as <code>action.places</code>.</p>
  
  <p>I'd like to combine this only with new data coming in and return a new map of the updated places. If I get an object of places and one of the places has the same lat/lon as a previous place, I don't want to append that data.</p>
  
  <pre><code>{
    places: [
      {
        place: {
          lat: 123,
          lon: 321,
        }
      },
      {
        place: {
          lat: 432,
          lon: 234,
        }
      },
    ]
  }
  </code></pre>
  
  <p>How would I do this with a map? Currently I'm appending this way but everything is duplicated, I can't find a non messy solution:</p>
  
  <pre><code>case RECEIVE_PLACES:
    return state
      .set('places', state.get('places').update(fromJS(action.places)))
  </code></pre>
  
  <p>Ideally I'd set the key of each obj to [<code>${lat},${lon}</code>] (only unique way to identify a place), that way I'd only have to iterate over the keys to get my places.</p>