The speed at which the mass of a body will double depends on the specific scenario and the physical processes involved. In classical physics, the mass of an object is typically considered to be constant, regardless of its speed. However, if we consider relativistic effects, such as the increase in mass with speed as described by Einstein's theory of special relativity, the situation changes.
According to special relativity, as an object approaches the speed of light, its relativistic mass increases. However, it is important to note that the mass increase is a consequence of the energy associated with the object's motion, rather than the object itself gaining more matter.
To precisely determine the speed at which the mass of a body will double due to relativistic effects, we need to know its initial mass, the reference frame in which the doubling is measured, and the exact equations involved. However, in general, the increase in mass becomes significant only when approaching velocities close to the speed of light (299,792,458 meters per second in a vacuum). At such extreme speeds, the energy required to double the mass of an object would be immense and would involve relativistic equations beyond the scope of a simple formula.
Therefore, without further details or constraints, it is not possible to provide a specific speed at which the mass of a body will double.