New Roof on Top of Old Shingles
You should never install new shingles over the old shingle roof system.
Your roof system will not be eligible for a warranty if you do not remove the existing shingle roof system before installing the new system. Installing over the old shingles can also cause unusual shapes to form in your new roof system and the roof system will not look smooth and level.
Rot, Moisture, and Accelerated Roof Aging
If the old roof system is not removed it will not be possible to inspect the plywood deck sheathing for rot or deterioration that can become a much larger problem in the future. Moisture from the previous roof system can be trapped under the new system and the increased surface temperatures from having two layers of shingles can accelerate roof aging.