This is probably a common newbie problem but I can't seem to get it to work by following other replies about this matter.

I am getting weird results trying to extrude four faces in and it seems I might have extruded twice and gotten double faces but when I try to delete them I get a hole and "Mesh/fill hole" doesn't work. Displaying Face centers seems to indicate I have double faces though but the poly counter says otherwise?

How do I delete these double faces so I can do an inwards extrude without any "walls" between these four faces (I did check "Edit Mesh/Keep faces together" but that didn't help either...)?
Any help greatly appriciated.