How to reprouduce:
1. Create a new Unity project
2. In the Project window, Create -> UI Toolkit -> TSS Theme File
3. Select the created TSS Theme File
4. In the Inspector, under the "Inherited Themes" click "+" -> Select the Asset 
5. Add the second element to the list and try assigning the same theme as the first time

Expected results: The same theme can't be assigned
Actual results: The same theme can be assigned and applying it doesn't result in any warnings or errors

Reproducible with: 2021.3.46f1, 2022.3.53f1, 6000.0.28f1, 6000.1.0a5

Reproducible on: Windows 10