Fix provided by Piotr Makal in patch D14204. This patch fixes volume grid duplication which was occurring during importing USD files. This was caused by calling BKE_volume_grid_add twice per grid (excluding 'density' grid) for the same Volume object: (1) in USDVolumeReader::read_object_data and (2) later in BKE_volume_load. Differential Revision: https://developer.blender.org/D14204