Steps to reproduce:

• Create a 3D project
• In Content Library - Materials,  add an instance of e.g. Brick material to the project
• In Navigator, right click the brick material and select "Add to Content Library"
• In Content Library - Materials. add another instance of the Brick material to the project
• -> "Item could not be created" error seen in Navigator for the second instance of the brick material. Also adding other materials from Materials tab then fails with the same error.