### 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)

d["preview_sun_env"] = pd;
	}
	return d;
}
void Node3DEditor::set_state(const Dictionary &p_state) {
	Dictionary d = p_state;
	if (d.has("snap_enabled")) {
		snap_enabled = d["snap_enabled"];
		tool_option_button[TOOL_OPT_USE_SNAP]->set_pressed(d["snap_enabled"]);
	}
	if (d.has("translate_snap")) {
		snap_translate_value = d["translate_snap"];
	}
	if (d.has("rotate_snap")) {
		snap_rotate_value = d["rotate_snap"];
	}
	if (d.has("scale_snap")) {
		snap_scale_value = d["scale_snap"];
	}
	_snap_update();
	if (d.has("local_coords")) {
		tool_option_button[TOOL_OPT_LOCAL_COORDS]->set_pressed(d["local_coords"]);
		update_transform_gizmo();
	}
	if (d.has("viewport_mode")) {
		int vc = d["viewport_mode"];
		if (vc == 1) {
			_menu_item_pressed(MENU_VIEW_USE_1_VIEWPORT);
		} else if (vc == 2) {
			_menu_item_pressed(MENU_VIEW_USE_2_VIEWPORTS);