Yes, they stamp your hand so you can come and go. However, if you are parking there, I don't think you can come and go in the parking lot without paying again. We stayed at a hotel with a shuttle specifically for this reason.
There is a mall a 5 minute walk away from the SP's parking lot, you could also park there if you need to go in and out and won't have to pay twice for parking. I don't remember if they give you a sticker or something when you first park your car.
Re: Sesame Place Question