### 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)

focused_axis = -2;
	queue_redraw();
}
void ViewportRotationControl::set_viewport(Node3DEditorViewport *p_viewport) {
	viewport = p_viewport;
}
void Node3DEditorViewport::_view_settings_confirmed(real_t p_interp_delta) {
	
	
	cursor.fov_scale = 1.0;
	_update_camera(p_interp_delta);
}
void Node3DEditorViewport::_update_navigation_controls_visibility() {
	bool show_viewport_rotation_gizmo = EDITOR_GET("editors/3d/navigation/show_viewport_rotation_gizmo") && (!previewing_cinema && !previewing_camera);
	rotation_control->set_visible(show_viewport_rotation_gizmo);
	bool show_viewport_navigation_gizmo = EDITOR_GET("editors/3d/navigation/show_viewport_navigation_gizmo") && (!previewing_cinema && !previewing_camera);
	position_control->set_visible(show_viewport_navigation_gizmo);
	look_control->set_visible(show_viewport_navigation_gizmo);
}
void Node3DEditorViewport::_update_camera(real_t p_interp_delta) {
	bool is_orthogonal = camera->get_projection() == Camera3D::PROJECTION_ORTHOGONAL;
	Cursor old_camera_cursor = camera_cursor;
	camera_cursor = cursor;
	if (p_interp_delta > 0) {
		
		
		if (is_freelook_active()) {
			
			
			const real_t inertia = EDITOR_GET("editors/3d/freelook/freelook_inertia");