Duplicated UI Toolkit elements lose hierarchical structure when elements in different hierarchy levels are duplicated

Steps to reproduce:

1. Create and open a new project
2. Create and open a new UI Document file
3. Add two Visual Elements in the UI Document
4. Add a child to each Visual Element, and expand the whole Hierarchy
5. Shift-select the last three elements
6. Press CTRL + D to duplicate
7. Observe the UI Builder's Hierarchy

Actual results: Hierarchical structure is lost: There are 3 children (1 grandchild) under the first Visual Element, and in total, there are only two top-level elements

Expected results: Hierarchical structure is kept: There are two children (no grandchildren) under the first Visual Element, and in total, there are three top-level elements

Reproducible with versions: 2023.1.0a10, 6000.0.73f1, 6000.3.13f1, 6000.4.3f1, 6000.5.0b4, 6000.6.0a3

Tested on (OS): Windows 11 Pro (25H2)