Reference:Double Illuminate Object Modifier

Syntax:

OBJECT {
  [OBJECT_ITEMS...]
  double_illuminate
  }

A surface has two sides; usually, only the side facing the light source is illuminated, the other side remains in shadow. When double_illuminate is used, the other side is also illuminated.
This is useful for simulating effects like translucency (as in a lamp shade, sheet of paper, etc).

Note: Using double_illuminate only illuminates both sides of the same surface, so on a sphere, for example, you will not see the effect unless the sphere is either partially transparent, or if the camera is inside and the light source outside of the sphere (or vise versa).