When 1.0 mol each of H2(g) and I2(g) are mixed at a certain high temperature, they react

Question:

When 1.0 mol each of H2(g) and I2(g) are mixed at a certain high temperature, they react to give a final mixture consisting of 0.5 mol each of H2(g) and I2(g) and 1.0 mol HI(g). Why do you obtain the same final mixture when you bring 2.0 mol HI(g) to the same temperature?