### 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)

view_menu->get_popup()->set_item_tooltip(wireframe_idx, unsupported_tooltip);
		view_menu->get_popup()->set_item_disabled(overdraw_idx, true);
		view_menu->get_popup()->set_item_tooltip(overdraw_idx, unsupported_tooltip);
		view_menu->get_popup()->set_item_disabled(shadeless_idx, true);
		view_menu->get_popup()->set_item_tooltip(shadeless_idx, unsupported_tooltip);
	}
	register_shortcut_action("spatial_editor/freelook_left", TTR("Freelook Left"), Key::A, true);
	register_shortcut_action("spatial_editor/freelook_right", TTR("Freelook Right"), Key::D, true);
	register_shortcut_action("spatial_editor/freelook_forward", TTR("Freelook Forward"), Key::W, true);
	register_shortcut_action("spatial_editor/freelook_backwards", TTR("Freelook Backwards"), Key::S, true);
	register_shortcut_action("spatial_editor/freelook_up", TTR("Freelook Up"), Key::E, true);
	register_shortcut_action("spatial_editor/freelook_down", TTR("Freelook Down"), Key::Q, true);
	register_shortcut_action("spatial_editor/freelook_speed_modifier", TTR("Freelook Speed Modifier"), Key::SHIFT);
	register_shortcut_action("spatial_editor/freelook_slow_modifier", TTR("Freelook Slow Modifier"), Key::ALT);
	ED_SHORTCUT("spatial_editor/lock_transform_x", TTR("Lock Transformation to X axis"), Key::X);
	ED_SHORTCUT("spatial_editor/lock_transform_y", TTR("Lock Transformation to Y axis"), Key::Y);
	ED_SHORTCUT("spatial_editor/lock_transform_z", TTR("Lock Transformation to Z axis"), Key::Z);
	ED_SHORTCUT("spatial_editor/lock_transform_yz", TTR("Lock Transformation to YZ plane"), KeyModifierMask::SHIFT | Key::X);
	ED_SHORTCUT("spatial_editor/lock_transform_xz", TTR("Lock Transformation to XZ plane"), KeyModifierMask::SHIFT | Key::Y);
	ED_SHORTCUT("spatial_editor/lock_transform_xy", TTR("Lock Transformation to XY plane"), KeyModifierMask::SHIFT | Key::Z);
	ED_SHORTCUT("spatial_editor/cancel_transform", TTR("Cancel Transformation"), Key::ESCAPE);
	ED_SHORTCUT("spatial_editor/instant_translate", TTR("Begin Translate Transformation"));
	ED_SHORTCUT("spatial_editor/instant_rotate", TTR("Begin Rotate Transformation"));
	ED_SHORTCUT("spatial_editor/instant_scale", TTR("Begin Scale Transformation"));
	preview_camera = memnew(CheckBox);
	preview_camera->set_text(TTR("Preview"));
	preview_camera->set_shortcut(ED_SHORTCUT("spatial_editor/toggle_camera_preview", TTR("Toggle Camera Preview"), KeyModifierMask::CMD_OR_CTRL | Key::P));
	vbox->add_child(preview_camera);
	preview_camera->set_h_size_flags(0);
	preview_camera->hide();