How to reproduce:
1. Create a new project
2. Go to the menu bar: GameObject > UI > Legacy > Button
3. Drag the "Button (Legacy)" GameObject from the Hierarchy to the Assets folder in the Project panel to create a Prefab
4. In the Hierarchy right-click the "Button (Legacy)" GameObject and go to: Prefab > Open Asset in Context
5. Select the greyed out root of the Prefab and press Ctrl+D to duplicate it
6. Save the Prefab and return to the scene view
7. Repeat step 4
8. Observe the Prefab's content in the Hierarchy
Expected result: Either the root of the Prefab cannot be copied or the changes made are saved with the prefab
Actual result: The root Prefab's in context view is duplicated but the duplicates aren't saved
Reproducible with: 2019.4.39f1, 2020.3.35f1, 2021.3.4f1, 2022.1.3f1, 2022.2.0a15
Reproducible on: Ubuntu 20.04, Windows 10
Note: The same behavior is reproducible when copying using Ctrl+C/X and Ctrl+V