title: React Native FlatList renders data twice when using extraData
body: <p>I am using React Native with firebase (without Redux or anything similar).
  I have a FlatList listening to changes with firebase by using the 'on' method.
  The FlatList ist listening to changes but when new Data gets rendered it renders the new data and the old data aswell. So I have the old data and then the old the data with the new data attached.</p>
  
  <p>I retrieve the data like this:</p>
  
  <pre><code>firebase.database().ref(`/thisThat/${uid}/arguments/this/`).orderByChild('timestamp')
        .on('value', (snapshot) =&gt; {
          snapshot.forEach((child) =&gt; {
              thisArr.push(child.val());
          });
        this.setState({ thisArguments: thisArr });
      });
  </code></pre>
  
  <p>I am doing it this way because I want it to be ordered (source: <a href="https://stackoverflow.com/a/33897213/5075510">https://stackoverflow.com/a/33897213/5075510</a>)</p>
  
  <p>My FlatList looks like this:</p>
  
  <pre><code>&lt;FlatList
    data={this.state.thisArguments}
    renderItem={this.renderItem}
    keyExtractor={(item, index) =&gt; index}
    ListEmptyComponent={this.emptyList}
    extraData={this.state}
   /&gt;
  </code></pre>
  
  <p>I think it propably has to do with the way I am retrieving the data. When I am doing it without the forEach everything works fine but it won't render ordered.
  Thank you in advance for your help!</p>