A number of factors come into play in making this decision. Older shingles tend to cup as they dry out and age. The new layer will follow the contour of the existing shingles which may cause your new roof to look bumpy or uneven. Also, the existing rows of shingles must have been run straight when installed. Again, the new layer will follow the old pattern and may lead to a visually unacceptable appearance.
Of greater concern is the inability to inspect the roof deck for heat damage, rot or loose decking. While the wood can be inspected from inside the attic space for broken boards or water damage, loose decking can not be determined without removing the shingles and existing felt paper. As wood becomes moist from humid air and condensation that collects in the attic space, it may swell slightly. As it dries out, the wood returns to its natural position. Over the course of 20 years, this swelling and relaxing of the wood can lift the decking nails slightly which may prevent the wood from laying flat.
This will be the only opportunity for the life of your new roof to address this condition. In general, removing the existing layer of shingles will add about $1000.00 to the cost of a roof replacement for a typical Northern Virginia home.