The escape velocity from the earth is about 11km /s. The escape velocity from a planet having twice the radius and the same mean density as the earth, is : 

22 km/s

\(\text{Escape velocity : } v_e = \sqrt{\frac{2Gm}{R}}\)

\(v_e = \sqrt{\frac{8}{3}\pi \rho G R^2}\)

\(v_e \propto R \) if :

\(\rho = constant\).

Since, the planet has double the radius in comparison to earth therefore, the escape velocity becomes twice i.e. 22 km/s.