### 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)

settings_fov->set_value(EDITOR_GET("editors/3d/default_fov"));
	settings_znear->set_value(EDITOR_GET("editors/3d/default_z_near"));
	settings_zfar->set_value(EDITOR_GET("editors/3d/default_z_far"));
	snap_translate_value = EditorSettings::get_singleton()->get_project_metadata("3d_editor", "snap_translate_value", 1);
	snap_rotate_value = EditorSettings::get_singleton()->get_project_metadata("3d_editor", "snap_rotate_value", 15);
	snap_scale_value = EditorSettings::get_singleton()->get_project_metadata("3d_editor", "snap_scale_value", 10);
	_snap_update();
	for (uint32_t i = 0; i < VIEWPORTS_COUNT; i++) {
		viewports[i]->reset();
	}
	if (origin_instance.is_valid()) {
		RenderingServer::get_singleton()->instance_set_visible(origin_instance, true);
	}
	view_menu->get_popup()->set_item_checked(view_menu->get_popup()->get_item_index(MENU_VIEW_ORIGIN), true);
	for (int i = 0; i < 3; ++i) {
		if (grid_enable[i]) {
			grid_visible[i] = true;
		}
	}
	for (uint32_t i = 0; i < VIEWPORTS_COUNT; i++) {
		viewports[i]->view_menu->get_popup()->set_item_checked(view_menu->get_popup()->get_item_index(Node3DEditorViewport::VIEW_AUDIO_LISTENER), i == 0);
		viewports[i]->viewport->set_as_audio_listener_3d(i == 0);
	}
	view_menu->get_popup()->set_item_checked(view_menu->get_popup()->get_item_index(MENU_VIEW_GRID), true);
	grid_enabled = true;
	grid_init_draw = false;
}
void Node3DEditor::_sun_direction_draw() {