Inside of top?

The inner faces are totally unnecessary! The only times you need them are where they'd be visible, such as the straps in between the arm and torso. Easiest way to do it (that I know of- i'm no blender expert) is to select the faces that need an inside counterpart, create a duplicate (shift+d), and click without moving the mouse to place. The duplicate faces should still be selected so you can just flip the normals of those