In some scenarios, inconsistencies when dropping an element twice on a diagram

[pcdavid]

Steps to reproduce

1. Create a General View diagram
2. Create a PartDefinition directly on the diagram. It is visible as a node with no compartments visible.
3. From the Explorer, create a sub-part part1 inside the PartDefinition. It is not visible on the diagram.
4. On the diagram, reveal the "parts" compartment, which now shows part1.
5. From the Explorer, drop part1 on the diagram. It is already visible as an item, so nothing happens. This is expected/normal (there should be a feedback message, but that is another issue).
6. Repeat the previous step: this time the result is different, and a top-level node part1 is created (not even at the mouse drop position).

Disabling the Hide expose elements filter in the Explorer shows that although nothing seems to happen at step 5 from the end-user's point of view, we've actually created an "expose" element for part1, which probably explains (at least in part) the different behavior when we repeat the operation.

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