If you come back to your car and find a parking ticket on it, can you leave the car there because you already have a ticket or can the warden come back later and give you another one?
When I suggested that I did that, to a traffic-warden who had just placed one on my car in St James' Square. He said, "Listen mate, If it's not gone in 5 minutes, I'll have you wheel-clamped!" - a charming fellow.