### 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)

texture_extensions.find(extension)) {
					Ref<Resource> res = ResourceLoader::load(files[i]);
					if (res.is_null()) {
						continue;
					}
					Ref<PackedScene> scn = res;
					Ref<Mesh> mesh = res;
					Ref<Material> mat = res;
					Ref<Texture2D> tex = res;
					if (scn.is_valid()) {
						Node *instantiated_scene = scn->instantiate(PackedScene::GEN_EDIT_STATE_INSTANCE);
						if (!instantiated_scene) {
							continue;
						}
						memdelete(instantiated_scene);
					} else if (mat.is_valid()) {
						Ref<BaseMaterial3D> base_mat = res;
						Ref<ShaderMaterial> shader_mat = res;
						if (base_mat.is_null() && !shader_mat.is_null()) {
							break;
						}
						spatial_editor->set_preview_material(mat);
						break;
					} else if (mesh.is_valid()) {
						/
					} else if (tex.is_valid()) {
						Ref<StandardMaterial3D> new_mat = memnew(StandardMaterial3D);
						new_mat->set_texture(BaseMaterial3D::TEXTURE_ALBEDO, tex);
						spatial_editor->set_preview_material(new_mat);
						break;