Uh, limited understanding here but, is the process of determining redundantly rendered frames something that can only be done via another "seperate" gpu/processor(igpu)?
Can the process be handled by the cpu?
Can the process be handled by the same GPU already doing the rendering?
I'm quite sure it is already being handled by the CPU. IGPU is used just because it gives a "free" framebuffer that can be locked to 60fps (for the Virtual Vsync)