### 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()->add_shortcut(ED_GET_SHORTCUT("spatial_editor/left_view"), VIEW_LEFT);
	view_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("spatial_editor/right_view"), VIEW_RIGHT);
	view_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("spatial_editor/front_view"), VIEW_FRONT);
	view_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("spatial_editor/rear_view"), VIEW_REAR);
	view_menu->get_popup()->add_separator();
	view_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("spatial_editor/switch_perspective_orthogonal"), VIEW_SWITCH_PERSPECTIVE_ORTHOGONAL);
	view_menu->get_popup()->add_radio_check_item(TTR("Perspective"), VIEW_PERSPECTIVE);
	view_menu->get_popup()->add_radio_check_item(TTR("Orthogonal"), VIEW_ORTHOGONAL);
	view_menu->get_popup()->set_item_checked(view_menu->get_popup()->get_item_index(VIEW_PERSPECTIVE), true);
	view_menu->get_popup()->add_check_item(TTR("Auto Orthogonal Enabled"), VIEW_AUTO_ORTHOGONAL);
	view_menu->get_popup()->set_item_checked(view_menu->get_popup()->get_item_index(VIEW_AUTO_ORTHOGONAL), true);
	view_menu->get_popup()->add_separator();
	view_menu->get_popup()->add_check_shortcut(ED_SHORTCUT("spatial_editor/view_lock_rotation", TTR("Lock View Rotation")), VIEW_LOCK_ROTATION);
	view_menu->get_popup()->add_separator();
	/
	view_menu->get_popup()->add_radio_check_shortcut(ED_SHORTCUT("spatial_editor/view_display_normal", TTR("Display Normal")), VIEW_DISPLAY_NORMAL);
	view_menu->get_popup()->add_radio_check_shortcut(ED_SHORTCUT("spatial_editor/view_display_wireframe", TTR("Display Wireframe")), VIEW_DISPLAY_WIREFRAME);
	view_menu->get_popup()->add_radio_check_shortcut(ED_SHORTCUT("spatial_editor/view_display_overdraw", TTR("Display Overdraw")), VIEW_DISPLAY_OVERDRAW);
	view_menu->get_popup()->add_radio_check_shortcut(ED_SHORTCUT("spatial_editor/view_display_lighting", TTR("Display Lighting")), VIEW_DISPLAY_LIGHTING);
	view_menu->get_popup()->add_radio_check_shortcut(ED_SHORTCUT("spatial_editor/view_display_unshaded", TTR("Display Unshaded")), VIEW_DISPLAY_UNSHADED);
	view_menu->get_popup()->set_item_checked(view_menu->get_popup()->get_item_index(VIEW_DISPLAY_NORMAL), true);
	display_submenu->set_hide_on_checkable_item_selection(false);
	display_submenu->add_radio_check_item(TTR("Directional Shadow Splits"), VIEW_DISPLAY_DEBUG_PSSM_SPLITS);
	display_submenu->add_separator();
	display_submenu->add_radio_check_item(TTR("Normal Buffer"), VIEW_DISPLAY_NORMAL_BUFFER);
	display_submenu->add_separator();
	display_submenu->add_radio_check_item(TTR("Shadow Atlas"), VIEW_DISPLAY_DEBUG_SHADOW_ATLAS);
	display_submenu->add_radio_check_item(TTR("Directional Shadow Map"), VIEW_DISPLAY_DEBUG_DIRECTIONAL_SHADOW_ATLAS);
	display_submenu->add_separator();
	display_submenu->add_radio_check_item(TTR("Decal Atlas"), VIEW_DISPLAY_DEBUG_DECAL_ATLAS);