### 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)

Vector2 size = get_size();
			int h_sep = get_theme_constant(SNAME("separation"), SNAME("HSplitContainer"));
			int v_sep = get_theme_constant(SNAME("separation"), SNAME("VSplitContainer"));
			int mid_w = size.width * ratio_h;
			int mid_h = size.height * ratio_v;
			bool was_hovering_h = hovering_h;
			bool was_hovering_v = hovering_v;
			hovering_h = mm->get_position().x > (mid_w - h_sep / 2) && mm->get_position().x < (mid_w + h_sep / 2);
			hovering_v = mm->get_position().y > (mid_h - v_sep / 2) && mm->get_position().y < (mid_h + v_sep / 2);
			if (was_hovering_h != hovering_h || was_hovering_v != hovering_v) {
				queue_redraw();
			}
		}
		if (dragging_h) {
			real_t new_ratio = drag_begin_ratio.x + (mm->get_position().x - drag_begin_pos.x) / get_size().width;
			new_ratio = CLAMP(new_ratio, 40 / get_size().width, (get_size().width - 40) / get_size().width);
			ratio_h = new_ratio;
			queue_sort();
			queue_redraw();
		}
		if (dragging_v) {
			real_t new_ratio = drag_begin_ratio.y + (mm->get_position().y - drag_begin_pos.y) / get_size().height;
			new_ratio = CLAMP(new_ratio, 40 / get_size().height, (get_size().height - 40) / get_size().height);
			ratio_v = new_ratio;
			queue_sort();
			queue_redraw();
		}
	}
}
void Node3DEditorViewportContainer::_notification(int p_what) {