It took one of the greatest minds in history to just about understand this subject, so I'm going to pretend I understand it fully, because it is brain distorting. Your point is fairly valid in the frame of reference of the clone earth, but it's impossible to every reach the speed of light because the object would have to gain an infinite amount of energy, it would have infinite mass from our Earth.

Maybe reaching travelling through space at super-luminous speeds is possible by distorting space-time itself, by compressing the space-time continuum in front of you and expanding it behind. Instead of moving through space, you move space around you. I wonder if controlling gravity could provide this mechanism.

It's a mental subject, and I must say not one of my favourites because it starts the make me crazy. haha.