Steps to reproduce:

1. Create a new project using Universal 3D template
2. Go to Assets > Create > Shader Graph > URP > UI Shader Graph and open it
3. In the Blackboard, click "" and add "Color" 
4. Save and exit
5. Create a new UXML Document and open it in UI Builder
6. Add a Visual Element to the Hierarchy
7. In the Inspector, expand the Material foldout and assign the newly created Material from Shader Graph
8. Click on the "" button and select "Color"
9. Repeat 8th step
10. Observe the behavior

Actual results: The exact same property can be added an infinite amount of times. Modifying a duplicated property's value instantly resets it to the top property's value making other properties redundant

Expected results: Properties that have already been added to the Material Definition override list are hidden, disabled, or grayed out in the "+" dropdown menu

Reproducible with versions: 6000.3.12f1, 6000.4.1f1, 6000.5.0b2, 6000.6.0a2

Can’t test with versions: 6000.0.71f1 (Materials foldout not available)

Tested on (OS): Windows 11

Notes:
- This applies to all property types exposed in the Shader Graph blackboard