here are two circuits in parallel connecting points A and B) each consisting of half one side of the hexagon, a rhombus and then half another side of the hexagon, connected in series. The resistance of each rhombus is R, therefore the resistance of one circuit is 2R. Therefore the resistance of the whole frame is R. The fact that the vertices of the rhombi are linked at 0 plays no part in this, since the fact that the two circuits under consideration are identical means that the vertices of both rhombi are at the same potential. Therefore we can calculate the resistances of the two circuits without taking into consideration their link at 0.