Copy/Paste From 1 Model View to Another Results in Duplicate Elements

### Version of Archi, Operating System

I'm using Archi version 4.2.0 on a Mac running macOS Mojave Version 10.14.5
### Expected Behaviour

Copying/pasting elements from a view in one model to another view in a different model should only create 1 version of each element in the model tree.
### Actual Behaviour

Elements referenced 2+ times in a view are duplicated when copy/pasted into a new view and model.
### Steps to Reproduce the Behaviour
1. Create a view where an element is used 2+ times
2. Copy all the elements in that view
3. Paste the elements into a new view in a different model

Reproduce the issue in Archi 4.2.0 on macOS by creating a view with an element used multiple times, copying all elements, and pasting into a view in another model. Trace the copy/paste path for cross-model view content; done means each referenced element appears only once in the destination model tree while all view references remain intact.

Written by the indexing model from the issue text.