How to reproduce:
1. Create a new Unity project
2. In the Project window, right-click -> Create -> UI Toolkit -> UI Document
3. Open the created UI Document and and create the new .USS file
4. Create a new selector
5. Create another selector with the identical name
6. Observe the results

Expected results: A warning is shown to notify about creating the selector with the existing name
Actual results: A warning is not shown

Reproducible with: 6000.0.38f1, 6000.1.0b5, 6000.2.0a3

Reproducible on: macOS Sequoia 15.3 (M1)
Not reproducible on: No other environment tested

Note: While duplicate names are possible, it might be nice to give this warning for the less experienced users