FlatList calls `onEndReached` when it’s rendered

Try to implement onMomentumScrollBegin on FlatList :

constructor(props) {
    super(props);
    this.onEndReachedCalledDuringMomentum = true;
}

…

<FlatList
    ...
    onEndReached={this.onEndReached.bind(this)}
    onEndReachedThreshold={0.5}
    onMomentumScrollBegin={() => { this.onEndReachedCalledDuringMomentum = false; }}
/>

and modify your onEndReached

onEndReached = ({ distanceFromEnd }) => {
    if(!this.onEndReachedCalledDuringMomentum){
        this.fetchData();
        this.onEndReachedCalledDuringMomentum = true;
    }
}