PrefabUtility.InstantiatePrefab not working with PrefabType.ModelPrefabInstance?

I have a PrefabType.ModelPrefabInstance in my scene which I have lightmapped and on which I have changed some parameters. What I now want to do is duplicate this and retain all it's parameters.

If I hit Ctrl+d I get exactly what I want; a duplicated version of the original with all connections to the model prefab retained and all parameters intact.

If I try to do this via an editor script using Object.Instantiate it breaks the connection to the prefab.

If I try to do this via an editor script using PrefabUtility.InstantiatePrefab I get an InvalidCastException.

Is what I want to do possible with an editor script?

Thanks