Can new sheet flooring be installed over old flooring?
Yes and no. If the old floor is a rigid vinyl or linoleum and is firmly attached, you may be able to. If the surface has any embossing, such as faux tile appearance, the floor should be leveled with an appropriate leveling product. The leveler is trowelled onto the floor to fill the pattern, giving a smooth surface for the new floor. The major manufacturers all sell these products, but you will probably have to order it through a flooring dealer.
If you plan to install the new sheet flooring over wood strip flooring, you should install an underlayment of a minimum of 1/4" plywood underlayment to assure a rigid floor. Strip wood floors flex and move with seasonal changes, and any boards that rise or move may appear as lines through the floor, causing premature wear and/or tears in the new flooring.
If the old floor is loose-lay or perimeter bonded sheet flooring (glue only around the edges and along seams), and is curled, badly torn, or is a cushion-back style, you should strip it before installing a new sheet floor... or any type of floor for that matter!