If i export a Cube 1 and Cube 2 from unity i guess the GLTF exporter see they have the same geometry mesh (Unity Built In Cube Mesh) and it creates Cube 1 as a regular mesh and creates Cube 2 as an Instanced Mesh with its source set to Cube 1.
This is kool for just loading geometry that is enabled and visible and ready to go in the scene. But i DISABLE Cube 1 and Cube 2 so i can clone them using code. If i clone cube 1 and then clone cube 2 it freezes the browser.
I will try to make a playground to load a sample scene with Cube 1 and Cube 2 to show but i was wondering if you are aware of deep cloning and the clone source is an instanced mesh ???