If the value of the electric field in an electromagnetic wave were doubled then

Electromagnetic wave are waves formed as a result of the oscillatory activities involving the electric and the magnetic field.

However in an Electromagnetic wave, the electric field and magnetic field carry equal amounts of energy and the magnitude of the electric field is directly proportional to the magnitude of the magnetic field. This direct proportionality gives rise to the speed of light being the constant between the two fields.

When the electric field is doubled then an equal action is to be set for the magnetic field so it doesn't deviate from its main functions and characteristics.