For people using Sketchup, as already stated, it does create multiple faces, so you’ll have normals pointing out on both sides. This may be changed within the export, I’m not sure, but it also creates multiple vertices as well. Which is also problematic. You just end up with a mesh that has way more faces and vertices than needed.
If you are bringing in Sketchup models into Blender, make sure to go into Edit Mode within Blender, select all and click “Remove Doubles”, this will merge everything together that reside within the same space.