the air starts coming out of the vents after it is turned of

I have taken it so many times to get checked the dealer tells me nothing is wrong. Finally i take it in after the warranty is expired and they find out that there is something wrong. They said that they had to change out the air conditioner modular and i had to pay my deductible. Only to find that it is worse now than it was. They tell me it is normal that it is the condensation that has to be dried. Yet my 2 sister have the same car nothing wrong with theirs. My brother is an air conditioning specialist and said that is not normal. This is not normal finally my brother said call gmc corp and file a complaint if nothing is done than i would hire an attorney. First they deliver the wrong car and sell me the wrong car then this and other problems when does it stop.