How to reproduce:
1. Open the “UIToolkitBugsBundle.zip“ project
2. Open the “UIDocument“ in UI Builder
3. Expand both USS files in the StyleSheets tab
4. Drag the “.Selector1“ selector to another USS file
5. Select the “1class.uss“ header in StyleSheets tab
6. Observe the USS Preview at the bottom of UI Builder

Expected result: The USS preview is empty
Actual result: The USS preview contains the definition for “.Selector1“

Reproducible in: 6000.0.41f1, 6000.0.49f1, 6000.1.3f1, 6000.2.0b1
Not reproducible in: 2022.3.62f1, 6000.0.40f1

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

Workaround: Rename any one selector from each affected USS file (or update the selector in any way)

Notes:
- As a result, this makes the UI Builder falsely duplicate a selector when the project or the UI Document are reopened
- There have to be at least 2 USS files added to the UI Document