No, Plato was not the only ancient Greek philosopher who believed in reincarnation. Earlier and even more important in this respect is certainly Pythagoras who in turn had it most probably from his teacher Pherecydes of Syros. All Pythagoreans and all Platonists believed in reincarnation, and these were many. There certainly were also philosophers beyond these two groups who believed in it. - In contrast, Stoics and Epicureans were not in favour of reincarnation.