There have been a number of suggestions of workarounds for this sort of problem.

Ady will be using one (not sure which one yet).

To build a false Vader he has to have some of the attributes of the real one (be trained by Obi-Wan and failed, have a reason to wear a mask, killing Jedi a plus).

To make it less obvious he could appear with Anakin.

Redubbing Grevious is one obvious candidate.

If Obi-Wan is not allowed to train Anakin because of a previous pupil going bad and then having a cyborg Jedi killer in the story people will assume that the General is the fallen pupil who later gets repackaged as Lord Vader.

It's weird that Lucas didn't consider this.