For the cell $\mathrm{Zn}/{\mathrm{Zn}}^{2+}//{\mathrm{Cu}}^{2+}/\mathrm{Cu}$ if the concentration of ${\mathrm{Zn}}^{2+}\text{ and }{\mathrm{Cu}}^{2+}$ ions is doubled, the emf of the cell

For the galvanic cell , the Nernst equation is written as 

${\mathrm{E}}_{\mathrm{cell}}={\mathrm{E}}_{\mathrm{cell}}^{\mathrm{o}}-\frac{0.0591}{\mathrm{n} }\log \frac{\left[{\mathrm{Zn}}^{2+}\right]}{\left[{\mathrm{Cu}}^{2+}\right]}$

Hence E_cell remains same, when conc. of both [Zn²⁺]  and  [Cu ²⁺] is doubled.