### 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)

add_child(c);
	c->set_anchors_and_offsets_preset(Control::PRESET_FULL_RECT);
	viewport = memnew(SubViewport);
	viewport->set_disable_input(true);
	c->add_child(viewport);
	surface = memnew(Control);
	SET_DRAG_FORWARDING_CD(surface, Node3DEditorViewport);
	add_child(surface);
	surface->set_anchors_and_offsets_preset(Control::PRESET_FULL_RECT);
	surface->set_clip_contents(true);
	camera = memnew(Camera3D);
	camera->set_disable_gizmos(true);
	camera->set_cull_mask(((1 << 20) - 1) | (1 << (GIZMO_BASE_LAYER + p_index)) | (1 << GIZMO_EDIT_LAYER) | (1 << GIZMO_GRID_LAYER) | (1 << MISC_TOOL_LAYER));
	viewport->add_child(camera);
	camera->make_current();
	surface->set_focus_mode(FOCUS_ALL);
	VBoxContainer *vbox = memnew(VBoxContainer);
	surface->add_child(vbox);
	vbox->set_offset(SIDE_LEFT, 10 * EDSCALE);
	vbox->set_offset(SIDE_TOP, 10 * EDSCALE);
	view_menu = memnew(MenuButton);
	view_menu->set_flat(false);
	view_menu->set_h_size_flags(0);
	view_menu->set_shortcut_context(this);
	vbox->add_child(view_menu);
	display_submenu = memnew(PopupMenu);
	view_menu->get_popup()->set_hide_on_checkable_item_selection(false);
	view_menu->get_popup()->add_child(display_submenu);
	view_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("spatial_editor/top_view"), VIEW_TOP);
	view_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("spatial_editor/bottom_view"), VIEW_BOTTOM);