In your code, check if a variable, NotificationActive, is false. If false, then set it to true, and then send a notification if NotificationActive was false.
Then, you can just add a delay to set the NotificationActive back to false.
You might want to keep a table of the configuration so you can keep the Duration field of it. Once config.Duration + 1 time passes (the extra 1 second is just a grace period, could be lowered or raised depending on how frequently you want the notifications to appear) you can then send another notification.