assuming we want the frequency to double,

going by this formula: en.wikipedia.org/wiki/Redshift

Image

with the wavelength ratio 1 + z we want 1 + z = 1/2 so v/c = -3/5 c = -0.6c if i calculated correctly (i did this in my head without a napkin), where the source moves away from the observer with velocity v. so it would have to move towards you quite rapidly.