A quick internet search will expose many articles that say this may not be a good idea from a roofing standpoint, at least not when putting new shingles over old roofing materials, especially shingles. The list of reasons include: 
•    The added weight of the double layer of roofing may exceed the structural design of the deck and/or roof framing.
•    Typical asphaltic shingles are designed to be installed over a flat deck, not a rough, uneven surface such as old shingles.
•    Not exposing the underlying roof deck prohibits it from being inspected from outside to identify potential leaks.
•    The new roof warranty is reduced or voided.

 These are all sound reasons, and an additional restriction may be that old roofing may be required to be removed prior to applying new roofing by local building regulations.
Installation of a new metal retrofit roof system over an old roof is an entirely different scenario. Such systems were specifically created for this purpose and take many of these factors into account in their design. This paper will educate a small building owners or homeowners on the advantages of retrofit metal roofing systems when applied over existing built-up roofs.