“When a body is moving with uniform speed on a rough surface under the application of a constant external force and an equal and opposite frictional force, the work done by both the forces should be equal. as a result, the net work done by both the forces should be zero. however, we find that heat energy is also evolved in this process.where does this extra energy come from?”
The work done is against friction only and if there is no friction, a force is not required to keep a body in uniform motion. The work done against friction appears as heat.