### 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)

Ref<InputEventMouseMotion> mm = p_event;
	if (mm.is_valid() && (mm->get_button_mask().has_flag(MouseButtonMask::LEFT))) {
		sun_rotation.x += mm->get_relative().y * (0.02 * EDSCALE);
		sun_rotation.y -= mm->get_relative().x * (0.02 * EDSCALE);
		sun_rotation.x = CLAMP(sun_rotation.x, -Math_TAU / 4, Math_TAU / 4);
		sun_angle_altitude->set_value(-Math::rad_to_deg(sun_rotation.x));
		sun_angle_azimuth->set_value(180.0 - Math::rad_to_deg(sun_rotation.y));
		_preview_settings_changed();
	}
}
void Node3DEditor::_sun_direction_angle_set() {
	sun_rotation.x = Math::deg_to_rad(-sun_angle_altitude->get_value());
	sun_rotation.y = Math::deg_to_rad(180.0 - sun_angle_azimuth->get_value());
	_preview_settings_changed();
}
Node3DEditor::Node3DEditor() {
	gizmo.visible = true;
	gizmo.scale = 1.0;
	viewport_environment = Ref<Environment>(memnew(Environment));
	VBoxContainer *vbc = this;
	custom_camera = nullptr;
	singleton = this;
	editor_selection = EditorNode::get_singleton()->get_editor_selection();
	editor_selection->add_editor_plugin(this);
	snap_enabled = false;
	snap_key_enabled = false;
	tool_mode = TOOL_MODE_SELECT;
	camera_override_viewport_id = 0;
	/
	/