### 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)

lock_rotation = p_lock_rotation;
	int idx = view_menu->get_popup()->get_item_index(VIEW_LOCK_ROTATION);
	view_menu->get_popup()->set_item_checked(idx, p_lock_rotation);
	if (p_lock_rotation) {
		locked_label->show();
	} else {
		locked_label->hide();
	}
}
Node3DEditorViewport::Node3DEditorViewport(Node3DEditor *p_spatial_editor, int p_index) {
	cpu_time_history_index = 0;
	gpu_time_history_index = 0;
	_edit.mode = TRANSFORM_NONE;
	_edit.plane = TRANSFORM_VIEW;
	_edit.snap = true;
	_edit.show_rotation_line = true;
	_edit.instant = false;
	_edit.gizmo_handle = -1;
	_edit.gizmo_handle_secondary = false;
	index = p_index;
	editor_selection = EditorNode::get_singleton()->get_editor_selection();
	orthogonal = false;
	auto_orthogonal = false;
	lock_rotation = false;
	message_time = 0;
	zoom_indicator_delay = 0.0;
	spatial_editor = p_spatial_editor;
	SubViewportContainer *c = memnew(SubViewportContainer);
	subviewport_container = c;
	c->set_stretch(true);