### 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)

Dictionary d;
	d["snap_enabled"] = snap_enabled;
	d["translate_snap"] = snap_translate_value;
	d["rotate_snap"] = snap_rotate_value;
	d["scale_snap"] = snap_scale_value;
	d["local_coords"] = tool_option_button[TOOL_OPT_LOCAL_COORDS]->is_pressed();
	int vc = 0;
	if (view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(MENU_VIEW_USE_1_VIEWPORT))) {
		vc = 1;
	} else if (view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(MENU_VIEW_USE_2_VIEWPORTS))) {
		vc = 2;
	} else if (view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(MENU_VIEW_USE_3_VIEWPORTS))) {
		vc = 3;
	} else if (view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(MENU_VIEW_USE_4_VIEWPORTS))) {
		vc = 4;
	} else if (view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(MENU_VIEW_USE_2_VIEWPORTS_ALT))) {
		vc = 5;
	} else if (view_menu->get_popup()->is_item_checked(view_menu->get_popup()->get_item_index(MENU_VIEW_USE_3_VIEWPORTS_ALT))) {
		vc = 6;
	}
	d["viewport_mode"] = vc;
	Array vpdata;
	for (int i = 0; i < 4; i++) {
		vpdata.push_back(viewports[i]->get_state());
	}
	d["viewports"] = vpdata;