### 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)

p->add_radio_check_shortcut(ED_SHORTCUT("spatial_editor/1_viewport", TTR("1 Viewport"), KeyModifierMask::CMD_OR_CTRL + Key::KEY_1), MENU_VIEW_USE_1_VIEWPORT);
	p->add_radio_check_shortcut(ED_SHORTCUT("spatial_editor/2_viewports", TTR("2 Viewports"), KeyModifierMask::CMD_OR_CTRL + Key::KEY_2), MENU_VIEW_USE_2_VIEWPORTS);
	p->add_radio_check_shortcut(ED_SHORTCUT("spatial_editor/2_viewports_alt", TTR("2 Viewports (Alt)"), KeyModifierMask::ALT + KeyModifierMask::CMD_OR_CTRL + Key::KEY_2), MENU_VIEW_USE_2_VIEWPORTS_ALT);
	p->add_radio_check_shortcut(ED_SHORTCUT("spatial_editor/3_viewports", TTR("3 Viewports"), KeyModifierMask::CMD_OR_CTRL + Key::KEY_3), MENU_VIEW_USE_3_VIEWPORTS);
	p->add_radio_check_shortcut(ED_SHORTCUT("spatial_editor/3_viewports_alt", TTR("3 Viewports (Alt)"), KeyModifierMask::ALT + KeyModifierMask::CMD_OR_CTRL + Key::KEY_3), MENU_VIEW_USE_3_VIEWPORTS_ALT);
	p->add_radio_check_shortcut(ED_SHORTCUT("spatial_editor/4_viewports", TTR("4 Viewports"), KeyModifierMask::CMD_OR_CTRL + Key::KEY_4), MENU_VIEW_USE_4_VIEWPORTS);
	p->add_separator();
	p->add_submenu_item(TTR("Gizmos"), "GizmosMenu");
	p->add_separator();
	p->add_check_shortcut(ED_SHORTCUT("spatial_editor/view_origin", TTR("View Origin")), MENU_VIEW_ORIGIN);
	p->add_check_shortcut(ED_SHORTCUT("spatial_editor/view_grid", TTR("View Grid"), Key::NUMBERSIGN), MENU_VIEW_GRID);
	p->add_separator();
	p->add_shortcut(ED_SHORTCUT("spatial_editor/settings", TTR("Settings...")), MENU_VIEW_CAMERA_SETTINGS);
	p->set_item_checked(p->get_item_index(MENU_VIEW_ORIGIN), true);
	p->set_item_checked(p->get_item_index(MENU_VIEW_GRID), true);
	p->connect("id_pressed", callable_mp(this, &Node3DEditor::_menu_item_pressed));
	gizmos_menu = memnew(PopupMenu);
	p->add_child(gizmos_menu);
	gizmos_menu->set_name("GizmosMenu");
	gizmos_menu->set_hide_on_checkable_item_selection(false);
	gizmos_menu->connect("id_pressed", callable_mp(this, &Node3DEditor::_menu_gizmo_toggled));
	/
	left_panel_split = memnew(HSplitContainer);
	left_panel_split->set_v_size_flags(SIZE_EXPAND_FILL);
	vbc->add_child(left_panel_split);
	right_panel_split = memnew(HSplitContainer);
	right_panel_split->set_v_size_flags(SIZE_EXPAND_FILL);
	left_panel_split->add_child(right_panel_split);
	shader_split = memnew(VSplitContainer);
	shader_split->set_h_size_flags(SIZE_EXPAND_FILL);