Are there a pros and cons to installing new shingles over the old layer of shingles?

The biggest drawback to installing new shingles over old ones is that it will most likely void your warranty. Another disadvantage to installing new shingles over old ones is that you deprive yourself of the opportunity to inspect your underlying roof--you may need to replace trusses, plywood sheeting and flashing. Old shingles can also hide mold. This could prove to be a time and money saving endeavour because you could potentially catch a small problem before it becomes a big one.

You may be able to add better insulation while you are at it, which will improve your energy costs.

*Two more negative points: 1)you are doubling the weight on the trusses or cut roof members. If you live in an area with high snow loads, you may overload the roof. 2) If you are sure that there is only 1 existing layer, you would probably be within the limits of any building code by adding another. However, some codes do not permit 3 layers due to a) the excess weight; and b) the difficulty in chopping through the roof by firefighters.