GUI，为什么组件绘图绘制了两次解决方法

import java.awt.*;import javax.swing.*;public class MainApp {    public static void main(String args[]){        GameControl gameControl = new GameControl();        Environment gameBackground = new Environment();        JFrame frame = new JFrame();        frame.add(gameBackground,BorderLayout.CENTER);        frame.setResizable(false);        frame.setVisible(true);         frame.setLocationRelativeTo(null);        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);        frame.setSize(475,494);        frame.setTitle("是男人就下一百层");    }}class Environment extends JPanel{    private Image background = new ImageIcon("backgroud.gif").getImage();    Environment(){          setLayout(null);        add(GameControl.boardPanel);  //注释了这行再运行的效果是我想达到的效果。。。        //想知道为什么会出现这个BUG，而不仅仅是怎么修复这个bug    }    public void paintComponent(Graphics g){        super.paintComponent(g);        g.drawImage(background,0,0,getWidth(),getHeight(),this);    }}class BoardPanel extends JPanel{    BoardPanel(){        setBounds(24,67,422,379);    }    public void paint(Graphics g){}}class GameControl implements Runnable{    public static BoardPanel boardPanel = new BoardPanel();     GameControl(){        new Thread(this).start();    }    public void run(){        while(true){            try{                Thread.sleep(30);            }            catch(InterruptedException e){            }            boardPanel.repaint();        }    }}