On Exit question

[NasaCow]

I am trying to control exiting to prevent work from getting loss and adding some basic code related to EXIT is having my program crash out with Error 10 - Duplicate definition (The error points to the label ShutDown). I tried to step through it to see how the program flows exactly but with no success. Is running the timer all the time to check a bad idea for complex programs? Getting it to work with a simple loop seems to be no problem, inserting into Grade Keeper seems to be breaking something...

'Disabling the default exit routinue
ExitFlag = EXIT
ON TIMER(1) GOSUB ShutDown
TIMER ON
...
ShutDown:
ExitFlag = EXIT
IF ExitFlag THEN SYSTEM
RETURN