在我的程序中，我希望能够按下一个按钮，打开一个新的tkinter窗口。我有这个工作，但我看到，当你再次按下按钮，相同的窗口打开。我如何防止这种情况？
代码：

def login_success():
    print('Login success')   #confirmation that Go button works, funtion will change

def login_screen():
    global screen
    root.geometry('300x350')
    root.title('Login')

    global username_login
    global password_login
    username_login = StringVar() #turns global variable into string
    password_login = StringVar()

    Label(text = 'TGUP' , bg = 'light blue', width ='300', height = '2', font =('Calibri',13)).pack()
    Label(text = '').pack()
    Button(text = 'Register', height='2',width='30', command=Reg).pack() #opens new window
    Label(text='').pack()
    Label(text = 'Login', fg = 'purple', font=10).pack()
    Label(text = 'Username').pack()
    username_entry1 = Entry(textvariable = username_login, bg='light grey').pack() #entry box for inputs
    Label(text = 'Password').pack()
    password_entry1 = Entry(textvariable = password_login, bg='light grey', show='*').pack()
    Button(text = 'Go', width = '10', height='2', fg='green', command=login_success ).pack()
    Label(text='').pack()
    Button(text='Exit', height='2',width='20', fg='red' ,command=root.destroy).pack()  #destroys window