### 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)

Node3D *sp = Object::cast_to<Node3D>(p_what);
	if (!sp) {
		return nullptr;
	}
	Node3DEditorSelectedItem *si = memnew(Node3DEditorSelectedItem);
	si->sp = sp;
	si->sbox_instance = RenderingServer::get_singleton()->instance_create2(
			selection_box->get_rid(),
			sp->get_world_3d()->get_scenario());
	si->sbox_instance_offset = RenderingServer::get_singleton()->instance_create2(
			selection_box->get_rid(),
			sp->get_world_3d()->get_scenario());
	RS::get_singleton()->instance_geometry_set_cast_shadows_setting(
			si->sbox_instance,
			RS::SHADOW_CASTING_SETTING_OFF);
	RS::get_singleton()->instance_geometry_set_cast_shadows_setting(
			si->sbox_instance_offset,
			RS::SHADOW_CASTING_SETTING_OFF);
	/
	/
	RS::get_singleton()->instance_set_layer_mask(si->sbox_instance, 1 << Node3DEditorViewport::GIZMO_EDIT_LAYER);
	RS::get_singleton()->instance_set_layer_mask(si->sbox_instance_offset, 1 << Node3DEditorViewport::GIZMO_EDIT_LAYER);
	RS::get_singleton()->instance_geometry_set_flag(si->sbox_instance, RS::INSTANCE_FLAG_IGNORE_OCCLUSION_CULLING, true);
	RS::get_singleton()->instance_geometry_set_flag(si->sbox_instance, RS::INSTANCE_FLAG_USE_BAKED_LIGHT, false);