title: React Native: Can't fix FlatList keys warning
body: <p>I'm trying to render a FlatList from json fetched from an api, but I keep getting this error:</p>
  
  <pre><code>Warning: Each child in an array or iterator should have a unique "key" prop.
  Check the render method of `VirtualizedList`.
  </code></pre>
  
  <p>Relevant code:</p>
  
  <pre><code>&lt;FlatList
    data={this.props.tunes}
    keyExtractor={(item, index) =&gt; item.id}
    renderItem={({item}) =&gt; {
      &lt;Item
        key={item.id}
        title={item.title}
        composer={item.composer}
        year={item.year}
      /&gt;
    }}
  /&gt;
  </code></pre>
  
  <p>I'm sure there is a simple fix for this, but after a few days of trying different things I haven't found it. Thanks for your help!</p>