Most local codes allow for an additional layer of roofing material to be installed. Problem is, you will not be able to correct any problems in the existing structure if there are any. An additional layer will cut the life expectancy of the new roof system. Your installer might not use long enough fasteners. Wall flashings will probably not be replaced. It will cost you a lot more next time to re-roof because of the additional labor cost to remove 2 layers.
Penny wise and dollar foolish, to just add another layer... just my