How to reproduce:
1. Open the “UIToolkitBugsBundle.zip“ project
2. Select the “.Selector1“ selector in the StyleSheets tab (must-do action)
3. Drag the “.Selector1“ selector onto the “VisualElement“ element in the Hierarchy
4. Press CTRL + Z (to undo style assignment)
5. Click on any selector in the StyleSheets tab
6. Observe the StyleSheets tab

Expected result: Nothing happens
Actual result: The “.Selector1“ selector is duplicated

Reproducible in: 2023.1.0a16, 6000.0.49f1, 6000.1.3f1, 6000.2.0b1
Not reproducible in: 2022.3.62f1, 2023.1.0a15

Reproduced on: Windows 11 Pro (24H2)
Not reproduced on: No other environment tested

Notes:
- If step 4 is skipped, the “.Selector1“ selector will move to another USS file instead of duplicating
- Often, the duplicated selector moves to the very bottom of the StyleSheet tab
- If the issue does not reproduce on the first attempt, try it the second time, though it should reproduce consistently if the steps are followed very carefully