Add existing element(s) shows more graphical nodes than expected

[AxelRICHARD]

1. Create a ActionFlowView diagram.
2. Create an ActionUsage action1 in the diagram.
3. Show its action flow compartment.
4. From the Explorer view, create a new sub-ActionUsage from action1.
5. In the diagram, open the palette on the action flow compartment and execute the "Add existing element(s)" menu item.

Expected results
The sub-ActionUsage is now visible in the action flow compartment.
Another node also exist on the diagram background (link to action1 through a composition node), but it is hidden.

Actual results
The sub-ActionUsage is now visible in the action flow compartment AND on the diagram background (link to action1 through a composition node).

You can also reproduce the problem by following the flashlight tutoriel, steps 4.17 and 4.18.