It shall be the duty of the owner or manager of every multiple-dwelling building containing two or more rental units, and the owner or occupant of every individually owned dwelling unit in any such multiple-unit building, to properly identify each dwelling unit by attaching identification numbers on or adjacent to each entrance to the individual units. The owner or manager of any multiple-dwelling building with common entrances containing two units or more, whether the units are rental or individually owned, shall provide signs, including directional arrows, easily identifying the location of each dwelling unit in the building. The signs shall be placed in an obvious location inside each entrance to the building.