I would be curious as to why you applied to the same company again if you think you will not accept the offer.
If you decline the second offer I would probably not expect to receive another offer from them.
A number of companies use internships to groom future employees. If you think you will not want to work for this company when you graduate, then I guess I would not worry too much about it. However, if this is a company you want to work for when you graduate, then I would not turn down the offer a second time.