Why do all the faces in my mesh turn into circles when adding a Subsurf modifier? i'm trying to use subdivision surfice modifier on models that i import from Sketchup. but they look like this:



before applying the modifier:



This happens with all models from Sketchup, even the simple box form to the left in the pictures. I've tried a bunch of different formats (3ds, fbx etc.) but it's all the same.

What might be the problem here?

I'm completely new to Blender, so please excuse my lack of knowledge.
All edges of the mesh are "split": every face has an  edge perfectly overlapping the edge of the adiacent face. If you select all in edit mode and remove doubles everything will turn fine.