AWT で再描画すべき領域が L 字型だと paint メソッドが2回呼ばれてしまう

import java.lang.ref.*;
import java.awt.*;
import java.awt.image.*;
import javax.swing.*;

public class JPainter extends JPanel {

    private Rectangle cacheRect = null;
    private Reference cacheRef = new SoftReference(null);

    public void paint(Graphics panelGraphics) {
        Rectangle visibleRect = getVisibleRect();
        Image cacheImage = (Image) cacheRef.get();
        if (cacheImage == null || !visibleRect.equals(cacheRect)) {
            cacheRect = visibleRect;
            cacheImage = createImage(cacheRect.width, cacheRect.height);
            cacheRef = new SoftReference(cacheImage);
            Graphics cacheGraphics = cacheImage.getGraphics();
            cacheGraphics.translate(-cacheRect.x, -cacheRect.y);
            cacheGraphics.setColor(Color.blue);
            cacheGraphics.drawLine(0, 0, getWidth(), getHeight());
            cacheGraphics.dispose();
        }
        panelGraphics.drawImage(cacheImage, cacheRect.x, cacheRect.y, this);
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        JPanel panel = new JPainter();
        panel.setPreferredSize(new Dimension(10000, 10000));
        JScrollPane scrollPane = new JScrollPane(panel);
        Container contentPane = frame.getContentPane();
        contentPane.add(scrollPane);
        frame.setSize(new Dimension(400, 300));
        frame.setVisible(true);
    }
}