- Use original (non-copied) references only when copying within 
  the same model resource.
- Otherwise, the copy in the destination resource will have 
  a reference to the original resource.
- NOTE
  - The resulting copy can have null-references which might
    not be handled correctly.
  - Hence, this is only an initial workaround, and a proper
    handling of copying references is needed.

Issue-Ref: 2845

Signed-off-by: Simon Barner <barner@fortiss.org>