### 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)

current_hover_gizmo = Ref<EditorNode3DGizmo>();
		current_hover_gizmo_handle = -1;
		current_hover_gizmo_handle_secondary = false;
		if (selected) {
			Vector<Ref<Node3DGizmo>> gizmos = selected->get_gizmos();
			for (int i = 0; i < gizmos.size(); i++) {
				Ref<EditorNode3DGizmo> seg = gizmos[i];
				if (!seg.is_valid()) {
					continue;
				}
				seg->set_selected(true);
			}
			selected->update_gizmos();
		}
	}
}
void Node3DEditor::_snap_changed() {
	snap_translate_value = snap_translate->get_text().to_float();
	snap_rotate_value = snap_rotate->get_text().to_float();
	snap_scale_value = snap_scale->get_text().to_float();
	EditorSettings::get_singleton()->set_project_metadata("3d_editor", "snap_translate_value", snap_translate_value);
	EditorSettings::get_singleton()->set_project_metadata("3d_editor", "snap_rotate_value", snap_rotate_value);
	EditorSettings::get_singleton()->set_project_metadata("3d_editor", "snap_scale_value", snap_scale_value);
}
void Node3DEditor::_snap_update() {
	snap_translate->set_text(String::num(snap_translate_value));
	snap_rotate->set_text(String::num(snap_rotate_value));
	snap_scale->set_text(String::num(snap_scale_value));
}
void Node3DEditor::_xform_dialog_action() {