"SelectorAccelerationCache ... association already exists" error is thrown when a style sheet is attached to a UI Document both directly and through @import

How to reproduce:

1. Open the attached “IN-152893” project
2. In the Project tab, open the “Assets/UI/Documents/Element1.uxml”
3. Close the UI Builder window
4. In the Project tab, open the “Assets/UI/Documents/Empty.uxml”
5. Observe the result in the Console of the Unity editor

Actual result: Error "SelectorAccelerationCache: Assets/UI/Styles/Unknown.uss / 22122:132272389 association already exists" is thrown in the Console
Expected result: No error is thrown in the Console when opening the "Empty.uxml" UI Document

Reproducible with: 6000.6.0a4, 6000.6.1f1, 6000.7.0a6
Not reproducible with: 6000.0.1f1, 6000.0.83f1, 6000.3.24f1, 6000.6.0a3

Reproducible on: Windows 11 Enterprise (25H2)
Not reproducible on: No other environments tested