How to Refresh A List containing Data from Rest API

Hello, i’m new to flutter. I’ve been trying manipulating my code to refresh the List of contents gotten from a rest API. I notice that new contents are being loaded upon triggering `RefreshIndicator()` but the full new items are being placed below the previously loaded contents. Please help. My full code below.

import 'dart:convert';
import 'dart:io';
import 'dart:async';
import 'package:cached_network_image/cached_network_image.dart';
import 'package:timeago/timeago.dart';
import 'package:flutter/material.dart';
import 'about.dart';
import 'fetchposts.dart';
import 'post.dart';
import 'post_details.dart';
import 'strings.dart';
import 'package:http/http.dart' as http;

class MyHomePage extends StatefulWidget {
 MyHomePage({Key key, this.title}) : super(key: key);

 // This widget is the home page of your application. It is stateful, meaning
 // that it has a State object (defined below) that contains fields that affect
 // how it looks.

 // This class is the configuration for the state. It holds the values (in this
 // case the title) provided by the parent (in this case the App widget) and
 // used by the build method of the State. Fields in a Widget subclass are
 // always marked "final".

 final String title;

 @override
 _MyHomePageState createState() => new _MyHomePageState();
}

enum IndicatorType { overscroll , refresh }

class _MyHomePageState extends State<MyHomePage> {
 bool _isRequestSent = false;
 bool _isRequestFailed = false;
 final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
 final GlobalKey<RefreshIndicatorState> _refreshIndicatorKey = new GlobalKey<RefreshIndicatorState>();
 List<Post> postList = [];

 @override
 void initState() {
 super.initState();
 _downloadData();
 }

 Future<Null> _handleRefresh() async {
 final Completer<Null> completer = new Completer<Null>();
 new Timer(const Duration(seconds: 3),() {completer.complete(null);});
 return completer.future.then((_){
 _downloadData();
 _scaffoldKey.currentState?.showSnackBar(new SnackBar(
 content: const Text('Refresh Complete'),
 action: new SnackBarAction(
 label: 'RETRY',
 onPressed: () {
 _refreshIndicatorKey.currentState.show();
 },
 )
 ));
 });
 }

 @override
 Widget build(BuildContext context) {
 // This method is rerun every time setState is called.
 //
 // The Flutter framework has been optimized to make rerunning build methods
 // fast, so that you can just rebuild anything that needs updating rather
 // than having to individually change instances of widgets.

 return Scaffold(
 key: _scaffoldKey,
 drawer: Drawer(
 child: ListView(
 padding: EdgeInsets.zero,
 children: <Widget>[
 DrawerHeader(
 child: Text('Gossipper'),
 decoration: BoxDecoration(color: Colors.white),
 ),
 ListTile(
 title: Text('Home'),
 onTap: () {
 Navigator.push(context,
 MaterialPageRoute(
 builder: (BuildContext context) => AboutUs()
 ),
 );
 },
 ),
 ListTile(
 title: Text('Forum'),
 onTap: () {

 },
 ),

 ],
 ),
 ),
 appBar: AppBar(
 // Here we take the value from the MyHomePage object that was created by
 // the App.build method, and use it to set our appbar title.
 title: Text(widget.title),
 backgroundColor: Colors.green,
 centerTitle: true,
 actions: <Widget>[
 new IconButton(
 icon: const Icon(Icons.refresh),
 tooltip: 'Refresh',
 onPressed: () {
 _refreshIndicatorKey.currentState.show();
 },
 )
 ],
 ),
 body: new Container(
 alignment: Alignment.center,
 child: !_isRequestSent
 // Request has not been sent, let's show a progress indicator
 ? new CircularProgressIndicator(backgroundColor: Colors.green,)

 // Request has been sent but did it fail?
 : _isRequestFailed
 // Yes, it has failed. Show a retry UI
 ? _showRetryUI()
 // No it didn't fail. Show the data
 : new RefreshIndicator(
 key: _refreshIndicatorKey,
 onRefresh: _handleRefresh,
 child: new Container(
 child: ListView.builder(
 itemCount: postList.length,
 scrollDirection: Axis.vertical,
 padding: EdgeInsets.all(8.0),
 itemBuilder: (BuildContext context, int index) {
 return _getPostWidgets(index);
 }),
 ),
 ),
 ),
 );
 }

 Widget _getPostWidgets(int index) {
 var post = postList[index];
 return new GestureDetector(
 onTap: () {
 openDetailsUI(post);
 },
 child: new Container(
 margin: const EdgeInsets.symmetric(horizontal: 5.0, vertical: 5.0),
 child: new Card(
 elevation: 3.0,
 child: new Row(
 children: <Widget>[
 new Container(
 width: 150.0,
 child: new CachedNetworkImage(
 imageUrl: post.thumbUrl,
 fit: BoxFit.cover,
 placeholder: new Icon(
 Icons.panorama,
 color: Colors.grey,
 size: 120.0,
 ),
 ),
 ),
 new Expanded(
 child: new Container(
 margin: new EdgeInsets.all(10.0),
 child: new Text(
 post.title,
 style: new TextStyle(color: Colors.black, fontSize: 18.0),
 ),
 )),

 ],
 ),
 ),
 ),
 );
 }

 void _downloadData() {
 MyHttpCall.getData().then((jsonData) {
 if(jsonData["results"] != null) {
 for (var jsonObject in jsonData["results"]) {
 var post = Post.getPostFrmJSONPost(jsonObject);
 postList.add(post);
 print(post);
 }
 setState(() => _isRequestSent = true);
 }
 }
 );
 }

 void handleRequestError() {
 setState(() {
 _isRequestSent = true;
 _isRequestFailed = true;
 });
 }

 Widget _showRetryUI() {
 return new Container(
 child: new Column(
 mainAxisAlignment: MainAxisAlignment.center,
 children: <Widget>[
 new Text(
 Strings.requestFailed,
 style: new TextStyle(fontSize: 16.0),
 ),
 new Padding(
 padding: new EdgeInsets.only(top: 10.0),
 child: new RaisedButton(
 onPressed: retryRequest,
 child: new Text(
 Strings.retry,
 style: new TextStyle(color: Colors.white),
 ),
 color: Theme.of(context).accentColor,
 splashColor: Colors.deepOrangeAccent,
 ),
 )
 ],
 ),
 );
 }

 void retryRequest() {
 setState(() {
 // Let's just reset the fields.
 _isRequestSent = false;
 _isRequestFailed = false;
 });
 }

 openDetailsUI(Post post) {
 Navigator.push(
 context,
 new MaterialPageRoute(
 builder: (BuildContext context) => new PostDetails(post)));
 }
}