如何使——用户名没有重复登录


import java.awt.*;
import java.awt.event.*;class TFrame extends Frame implements ActionListener,ItemListener
{
    Label l,l1,l2;
    List list;
    TextField t1,t2;
    Button b1,b2,b3;
    Panel p1,p2,panel;    public TFrame()
    {
       super("用户登录");
       this.setBounds(100,100,300,200);
       this.add(l=new Label("登录信息在这里显示",Label.LEFT),"North");
       list=new List();
       list.add("10000:10000");
       list.add("20101:03021");
       list.addItemListener(this);
       this.add(list,"West");
       
       panel=new Panel();
       p1=new Panel();
       p1.setLayout(new GridLayout(2,2));
       p1.add(l1=new Label("用户名："));
       p1.add(t1=new TextField(5));
       p1.add(l2=new Label("密  码："));
       p1.add(t2=new TextField(5));
       t2.setEchoChar('*');
       panel.add(p1);
       
       p2=new Panel();
       p2.add(b1=new Button("重置"));
       b1.addActionListener(this);
       p2.add(b2=new Button("登录"));
       b2.addActionListener(this);
       p2.add(b3=new Button("取消"));
       b3.addActionListener(this);
       panel.add(p2);
       
       this.add(panel,"Center");       
       this.addWindowListener(new WindowAdapter(){
          public void windowClosing(WindowEvent e)
          {
              System.exit(0);
          }
       });
       this.setVisible(true);
    }    public void actionPerformed(ActionEvent e) {
        if(e.getSource().equals(b3))
        {
            System.exit(0);
        }
        if(e.getSource().equals(b2))
        {
            String str=t1.getText()+":"+t2.getText();
            label:
           for(int i=0;i<list.getItemCount();i++)
           {
               if(list.getItem(i).equals(str))
               {
                  break label;
               }
           }
           list.add(str);
        }
        if(e.getSource().equals(b1))
        {
            t1.setText("");
            t2.setText("");
        }
    }    public void itemStateChanged(ItemEvent e) {
        
    }
    
}
public class T1
{
    public static void main(String[] args) {
        new TFrame();
    }
}
问题是：当点击登录按钮时，怎样可以实现如果已有用户已存在，则不添加！