OK, so, a couple of things, and I'll try to be more careful with my typing. Double shadows are going to happen when the light sources are set too far toward the sides. The lightbox is supposed to be a diffuser that would essentially eliminate that issue, which comes with "hard" source point lighting. If you have any other options to move the position of the lights, or the subject, so that the lights are a bit more "in front" and less "to the side" of the subject that issue may be resolved. When I'm shooting something my sources are placed at an angle of between 30˚ and 45˚ and outward and in front from the plane of the subject, pretty standard set up.
There are better tools for editing than using brightness, which is occasionally the right move, but more often is a blunt instrument. Try adjusting contrast using white point, black point, and gray point sliders, if you have them, the equivalent of a levels adjustment in Photoshop, and if not available in your kit of tools you can cheat a similar result by adjusting using both contrast and brightness, but less reliance on brightness. If you have a curve function, use that first. Much more control of how you adjust tonal levels in your image.