### 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)

if (sun_environ_updating) {
		return;
	}
	{ /
		Transform3D t;
		t.basis = Basis::from_euler(Vector3(sun_rotation.x, sun_rotation.y, 0));
		preview_sun->set_transform(t);
		sun_direction->queue_redraw();
		preview_sun->set_param(Light3D::PARAM_ENERGY, sun_energy->get_value());
		preview_sun->set_param(Light3D::PARAM_SHADOW_MAX_DISTANCE, sun_max_distance->get_value());
		preview_sun->set_color(sun_color->get_pick_color());
	}
	{ /
		sky_material->set_sky_energy_multiplier(environ_energy->get_value());
		Color hz_color = environ_sky_color->get_pick_color().lerp(environ_ground_color->get_pick_color(), 0.5).lerp(Color(1, 1, 1), 0.5);
		sky_material->set_sky_top_color(environ_sky_color->get_pick_color());
		sky_material->set_sky_horizon_color(hz_color);
		sky_material->set_ground_bottom_color(environ_ground_color->get_pick_color());
		sky_material->set_ground_horizon_color(hz_color);
		environment->set_ssao_enabled(environ_ao_button->is_pressed());
		environment->set_glow_enabled(environ_glow_button->is_pressed());
		environment->set_sdfgi_enabled(environ_gi_button->is_pressed());
		environment->set_tonemapper(environ_tonemap_button->is_pressed() ? Environment::TONE_MAPPER_FILMIC : Environment::TONE_MAPPER_LINEAR);
	}
}
void Node3DEditor::_load_default_preview_settings() {