python GPIO pins won't cleanup

I am a beginer with python and decided to give it a go on my raspberry pi. I learned python from a small book that covers the basics, I combined the demo to change the Tkinter window background color with a toggle button and the lighting up a LED to get this code:

#! /usr/bin/env python
import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BOARD); GPIO.setup(7, GPIO.OUT)
GPIO.output(7, False)
from Tkinter import*
window = Tk()
window.title("Relay Button")
window.configure(bg= "green")
btn_end= Button(window, text = "close", command=exit)
def tog():
        if ( GPIO.input(7) == True ):
                GPIO.output(7, False)
        else:
                GPIO.output(7, True)
btn_tog=Button( window, text="Switch", command=tog)
btn_end.pack(padx=100, pady=20)
btn_tog.pack(padx=100, pady=20)
window.mainloop()
GPIO.cleanup()

I get the error :

relaybutton.py:3: RuntimeWarning: This channel is already in use,
continuing anyway.  Use GPIO.setwarnings(False) to disable warnings.
  GPIO.setmode(GPIO.BOARD); GPIO.setup(7, GPIO.OUT)

I have no clue why I get this error, I have at the end the GPIO.cleanup().