### 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)

top_right_vbox->set_anchors_and_offsets_preset(PRESET_TOP_RIGHT, PRESET_MODE_MINSIZE, 10.0 * EDSCALE);
	top_right_vbox->set_h_grow_direction(GROW_DIRECTION_BEGIN);
	/
	top_right_vbox->set_custom_minimum_size(Size2(100, 0) * EDSCALE);
	/
	top_right_vbox->add_theme_constant_override("separation", 0);
	const int navigation_control_size = 150;
	position_control = memnew(ViewportNavigationControl);
	position_control->set_navigation_mode(Node3DEditorViewport::NAVIGATION_MOVE);
	position_control->set_custom_minimum_size(Size2(navigation_control_size, navigation_control_size) * EDSCALE);
	position_control->set_h_size_flags(SIZE_SHRINK_END);
	position_control->set_anchor_and_offset(SIDE_LEFT, ANCHOR_BEGIN, 0);
	position_control->set_anchor_and_offset(SIDE_TOP, ANCHOR_END, -navigation_control_size * EDSCALE);
	position_control->set_anchor_and_offset(SIDE_RIGHT, ANCHOR_BEGIN, navigation_control_size * EDSCALE);
	position_control->set_anchor_and_offset(SIDE_BOTTOM, ANCHOR_END, 0);
	position_control->set_viewport(this);
	surface->add_child(position_control);
	look_control = memnew(ViewportNavigationControl);
	look_control->set_navigation_mode(Node3DEditorViewport::NAVIGATION_LOOK);
	look_control->set_custom_minimum_size(Size2(navigation_control_size, navigation_control_size) * EDSCALE);
	look_control->set_h_size_flags(SIZE_SHRINK_END);
	look_control->set_anchor_and_offset(SIDE_LEFT, ANCHOR_END, -navigation_control_size * EDSCALE);
	look_control->set_anchor_and_offset(SIDE_TOP, ANCHOR_END, -navigation_control_size * EDSCALE);
	look_control->set_anchor_and_offset(SIDE_RIGHT, ANCHOR_END, 0);
	look_control->set_anchor_and_offset(SIDE_BOTTOM, ANCHOR_END, 0);
	look_control->set_viewport(this);
	surface->add_child(look_control);
	rotation_control = memnew(ViewportRotationControl);
	rotation_control->set_custom_minimum_size(Size2(80, 80) * EDSCALE);
	rotation_control->set_h_size_flags(SIZE_SHRINK_END);