Back EMF is computed on the basis of a magnetic flux and area; the same area, and
the same flux, doesn't generate a 'second' effect. The importance of the back EMF
in the operation of motors and transformers is in conservation of energy: any motor
power not taken up by the load, results in lessening the input energy delivered,
because an unloaded motor generates higher back-EMF than one which is doing work.
Effects of the back-EMF in a motor DO happen back at the powerhouse in the
generator there, where less energy is required to turn it, but that is
a different magnetic field, and a different area, and not generally a user
concern. Other effects of the back-EMF in a motor are seen in the
power meter for my house, of course: I find that quite gratifying.