When it comes time to install a new roof, most homeowners will decide whether to have the existing shingles removed before installing new ones. Most jurisdictions limit the number of layers allowed on a roof, in most places, that limit is two or three layers. In some places, it’s only one layer.
Installing new shingles over old has some disadvantages:
It will void any manufacturer’s warranty.
Shingles will dissipate heat more slowly, which will shorten their lifespan.
Shingles may radiate more heat into the living space, increasing cooling costs.
It adds extra weight and stress to the roof structure
This photo shows 3-layers of asphalt shingles, I highly recommend removing all layers of old shingles before re-roofing, most good roofers will agree.