As per my very little knowledge about computer graphics, I think the effect would be minimal. The reason is, given that the two screens has the same resolution, the pixel calculations are done exactly once and only the 'writing to framebuffer' operation is done twice given that the two screens has the same resolution, which is the last step of the graphics pipeline.

How much of a workload is the last step compared to the rest of the pipeline? Would the effects be minimal, i.e., If I duplicate the screens, would my not-so-powerful GPU have trouble maintaining a reasonable FPS rate when playing games due to duplicating the screens?