### Godot version

Master (4.2)

### System information

Windows 11 - Vulkan - Nvidia RTX 4070 - intel i5 13600KF

### Issue description

As you can see in the video when you try to add multiple scenes in to the 3D viewport (only with multiple scenes) the editor throws this error.

```
ERROR: Can't add child '@Node3D@18837' to '@SubViewport@9274', already has a parent '@SubViewport@9274'.
   at: (scene\main\node.cpp:1411)
```


Video: https://youtu.be/BVcCIigQJDw

This happens in the latest master (pulled on 2.11.2023).

### Steps to reproduce

Drag couple of the 3d mesh scenes into the 3D viewport and then select multiple scenes and drag them over and observer the error.

### Minimal reproduction project

[TestSceneDragAndDrop.zip](https://github.com/godotengine/godot/files/13253741/TestSceneDragAndDrop.zip)

line_bgn[a] = bgn_a;
				line_end[a] = end_a;
				ref_grid.set(idx, line_bgn);
				ref_grid.set(idx + 1, line_end);
				ref_grid_colors.set(idx, line_color);
				ref_grid_colors.set(idx + 1, line_color);
				ref_grid_normals.set(idx, normal);
				ref_grid_normals.set(idx + 1, normal);
				idx += 2;
			}
		}
		/
		grid[c] = RenderingServer::get_singleton()->mesh_create();
		Array d;
		d.resize(RS::ARRAY_MAX);
		d[RenderingServer::ARRAY_VERTEX] = grid_points[c];
		d[RenderingServer::ARRAY_COLOR] = grid_colors[c];
		d[RenderingServer::ARRAY_NORMAL] = grid_normals[c];
		RenderingServer::get_singleton()->mesh_add_surface_from_arrays(grid[c], RenderingServer::PRIMITIVE_LINES, d);
		RenderingServer::get_singleton()->mesh_surface_set_material(grid[c], 0, grid_mat[c]->get_rid());
		grid_instance[c] = RenderingServer::get_singleton()->instance_create2(grid[c], get_tree()->get_root()->get_world_3d()->get_scenario());
		/
		RenderingServer::get_singleton()->instance_set_visible(grid_instance[c], grid_visible[a]);
		RenderingServer::get_singleton()->instance_geometry_set_cast_shadows_setting(grid_instance[c], RS::SHADOW_CASTING_SETTING_OFF);
		RS::get_singleton()->instance_set_layer_mask(grid_instance[c], 1 << Node3DEditorViewport::GIZMO_GRID_LAYER);
		RS::get_singleton()->instance_geometry_set_flag(grid_instance[c], RS::INSTANCE_FLAG_IGNORE_OCCLUSION_CULLING, true);
		RS::get_singleton()->instance_geometry_set_flag(grid_instance[c], RS::INSTANCE_FLAG_USE_BAKED_LIGHT, false);
	}
}
void Node3DEditor::_finish_indicators() {