When 3D objects are drawn on the canvas and rendered, by default only polygons which face you are drawn. Pressing the Double button causes all polygons (including those facing away from you – the backfaces -) to be drawn. Pressing the Double button has no effect on the object’s true geometry, only on how it is drawn on the canvas.
The Flip Normals button reverses the order and direction of the polygons which compose the current 3D tool. If an imported 3D object appears to be inside-out, pressing the Flip button may fix this problem. Pressing the Flip button may sometimes appear to have no effect on the object.