### 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)

for (int i = 0; i < 3; i++) {
		move_gizmo[i]->surface_set_material(0, i == p_axis ? gizmo_color_hl[i] : gizmo_color[i]);
		move_plane_gizmo[i]->surface_set_material(0, (i + 6) == p_axis ? plane_gizmo_color_hl[i] : plane_gizmo_color[i]);
		rotate_gizmo[i]->surface_set_material(0, (i + 3) == p_axis ? rotate_gizmo_color_hl[i] : rotate_gizmo_color[i]);
		scale_gizmo[i]->surface_set_material(0, (i + 9) == p_axis ? gizmo_color_hl[i] : gizmo_color[i]);
		scale_plane_gizmo[i]->surface_set_material(0, (i + 12) == p_axis ? plane_gizmo_color_hl[i] : plane_gizmo_color[i]);
	}
}
void Node3DEditor::update_transform_gizmo() {
	int count = 0;
	bool local_gizmo_coords = are_local_coords_enabled();
	Vector3 gizmo_center;
	Basis gizmo_basis;
	Node3DEditorSelectedItem *se = selected ? editor_selection->get_node_editor_data<Node3DEditorSelectedItem>(selected) : nullptr;
	if (se && se->gizmo.is_valid()) {
		for (const KeyValue<int, Transform3D> &E : se->subgizmos) {
			Transform3D xf = se->sp->get_global_transform() * se->gizmo->get_subgizmo_transform(E.key);
			gizmo_center += xf.origin;
			if (count == 0 && local_gizmo_coords) {
				gizmo_basis = xf.basis;
			}
			count++;
		}