Depends on the color of the wall, but likely no. A matte black wall would absorb a lot of light, a matte white wall would reflect most of the light. Other colours would fall somewhere in the middle, reflecting some wavelengths and absorbing others. The only difference with a mirror is that it reflects light in a uniform fashion, whereas a painted wall will generally scatter reflected light. But scattered light still contributes to total light output! The only scenario where a mirror behind a lamp would come close to doubling light output would be if the wall we’re comparing against is painted with Vantablack or some other ultrablack paint that absorbs 99%+ of the light from the lamp.