### 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)

frame_time_gradient->get_color_at_offset(
								Math::remap(cpu_time, 0, 30, 0, 1)));
				gpu_time_label->set_text(vformat(TTR("GPU Time: %s ms"), rtos(gpu_time).pad_decimals(2)));
				/
				gpu_time_label->add_theme_color_override(
						"font_color",
						frame_time_gradient->get_color_at_offset(
								Math::remap(gpu_time, 0, 30, 0, 1)));
				const double fps = 1000.0 / gpu_time;
				fps_label->set_text(vformat(TTR("FPS: %d"), fps));
				/
				fps_label->add_theme_color_override(
						"font_color",
						frame_time_gradient->get_color_at_offset(
								Math::remap(fps, 110, 10, 0, 1)));
			}
			bool show_cinema = view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(VIEW_CINEMATIC_PREVIEW));
			cinema_label->set_visible(show_cinema);
			if (show_cinema) {
				float cinema_half_width = cinema_label->get_size().width / 2.0f;
				cinema_label->set_anchor_and_offset(SIDE_LEFT, 0.5f, -cinema_half_width);
			}
			if (lock_rotation) {
				float locked_half_width = locked_label->get_size().width / 2.0f;
				locked_label->set_anchor_and_offset(SIDE_LEFT, 0.5f, -locked_half_width);
			}
		} break;
		case NOTIFICATION_PHYSICS_PROCESS: {
			if (!update_preview_node) {
				return;