Reproduction steps:
1. Create a new Universal 3D template project
2. Create > Material
3. Select newly created Material and open Shader selection menu by clicking on the Shader dropdown in the Inspector
4. Select Test Shader (Custom > Test Shader)
5. Open Shader selection menu again and observe Custom section

Actual result: Test Shader is duplicated in Shader selection menu and Both Test Shaders show up as selected
Expected result: Test Shader is not duplicated

Reproducible with: 6000.0.67f1, 6000.3.8f1, 6000.4.0b8, 6000.5.0a6

Reproduced on: Windows 11, macOS Sequoia 26.2 (M4)