XMI-Import: imported elementes doubled in Hierarchical View (#125)

When I import from a XMI-File the elements in the hierarchical view are present twice. The elements that are subsidiary to the Activity seem to have no function and i cant move or delete them. The duplicates that are not subsidiary to the activity seem to be the real ones. I can move and delete them. If i delete the Activity, all of the subsidiary elements are deleted but also most of the duplicates are deleted.

The hierarchy looks like this:

- project

- model

- diagram

- activity

- element 1

- ...

- duplicate of element 1

- ...