link or maybe separation problem I have the following problem. at the beginning, i duplicated the "wheelbox" via duplicate link. first all was ok. but now i am coming to the point where i want to work on the inner/outer part of the object. but unfortunately, i cannot select Inner / outer wall. when i select (like u see here in the image) the outside wall of the right box it select also the inner wall of the left box. how can i separate this link?
Internally, when you create a linked duplicate, blender does not actually duplicate the object, but it has another reference to it, thus, when you edit the objects, you are editing all linked duplicates the same way. You need to break the link  between the two if you want to edit those elements separately. 

Since I am pretty bad at explaining these sort of things without demonstrating, here are two helpful links about duplicates and linked duplicates. 

I would advise you to look at these two links How to unlink a linked-duplicate? and Duplication -- Blender manual.