From Einstein's photoelectric equation, $eV_s = hf - \phi$, where $V_s$ is stopping potential, f is frequency, and $\phi$ is work function. If frequency is doubled, the new stopping potential is $eV'_s = h(2f) - \phi = 2(hf) - \phi = 2(eV_s + \phi) - \phi = 2eV_s + \phi$. Since $\phi > 0$, $V'_s > 2V_s$. The stopping potential becomes more than double.