Images loaded in ListView are randomly exchanged

I've implemented a ListView where an external image is populated in each position, but when I scroll on the screen the images change randomly.

This video shows exactly what happens: link

Note: There should be no duplicate images in the ListView.

DownloadTask:

public class DownloadTask extends AsyncTask<String, Void, Bitmap> {
    private final WeakReference<ImageView> imageView;

    public DownloadTask(ImageView thumbnail)
    {
        imageView = new WeakReference<ImageView>(thumbnail);
    }

    @Override
    protected Bitmap doInBackground(String... params)
    {

        try {
            InputStream strem = new URL(params[0]).openConnection().getInputStream();
            return BitmapFactory.decodeStream(strem);
        } catch (MalformedURLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        return null;
    }

    @Override
    protected void onPostExecute(Bitmap bitmap)
    {
        ImageView thumbnail = imageView.get();

        if (thumbnail != null)
        {
            if (bitmap != null)
            {
                thumbnail.setImageBitmap(bitmap);
            }
        }
    }
}

Adapter:

public class Adapter extends SimpleAdapter {
    public LayoutInflater inflater = null;

    public Adapter(Context context, List<? extends Map<String, ?>> data, int resource, String[] from, int[] to)
    {
        super(context, data, resource, from, to);

        inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }

    @Override
    public View getView(int position, View view, ViewGroup parent) {
        ViewHolder viewHolder;

        HashMap<String, Object> data = (HashMap<String, Object>) super.getItem(position);

        if(view == null){
            view = inflater.inflate(R.layout.clientes, parent, false);

            viewHolder = new ViewHolder();

            viewHolder.thumbnail = (ImageView) view.findViewById(R.id.thumbnail);
            viewHolder.title = (TextView)view.findViewById(R.id.title);
            viewHolder.content = (TextView)view.findViewById(R.id.content);

            view.setTag(viewHolder);
        }else{
            viewHolder = (ViewHolder)view.getTag();
        }

        new DownloadTask(viewHolder.thumbnail).execute(data.get("thumbnail").toString());

        viewHolder.title.setText(data.get("title").toString());
        viewHolder.content.setText(data.get("content").toString());

        return view;
    }

    static class ViewHolder{
        ImageView thumbnail;
        TextView title;
        TextView content;
    }
}