### 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)

sun_environ_settings->set_theme_type_variation("FlatButton");
	sun_environ_settings->connect("pressed", callable_mp(this, &Node3DEditor::_sun_environ_settings_pressed));
	main_menu_hbox->add_child(sun_environ_settings);
	main_menu_hbox->add_child(memnew(VSeparator));
	/
	preview_node = memnew(Node3D);
	preview_bounds = AABB();
	ED_SHORTCUT("spatial_editor/bottom_view", TTR("Bottom View"), KeyModifierMask::ALT + Key::KP_7);
	ED_SHORTCUT("spatial_editor/top_view", TTR("Top View"), Key::KP_7);
	ED_SHORTCUT("spatial_editor/rear_view", TTR("Rear View"), KeyModifierMask::ALT + Key::KP_1);
	ED_SHORTCUT("spatial_editor/front_view", TTR("Front View"), Key::KP_1);
	ED_SHORTCUT("spatial_editor/left_view", TTR("Left View"), KeyModifierMask::ALT + Key::KP_3);
	ED_SHORTCUT("spatial_editor/right_view", TTR("Right View"), Key::KP_3);
	ED_SHORTCUT("spatial_editor/orbit_view_down", TTR("Orbit View Down"), Key::KP_2);
	ED_SHORTCUT("spatial_editor/orbit_view_left", TTR("Orbit View Left"), Key::KP_4);
	ED_SHORTCUT("spatial_editor/orbit_view_right", TTR("Orbit View Right"), Key::KP_6);
	ED_SHORTCUT("spatial_editor/orbit_view_up", TTR("Orbit View Up"), Key::KP_8);
	ED_SHORTCUT("spatial_editor/orbit_view_180", TTR("Orbit View 180"), Key::KP_9);
	ED_SHORTCUT("spatial_editor/switch_perspective_orthogonal", TTR("Switch Perspective/Orthogonal View"), Key::KP_5);
	ED_SHORTCUT("spatial_editor/insert_anim_key", TTR("Insert Animation Key"), Key::K);
	ED_SHORTCUT("spatial_editor/focus_origin", TTR("Focus Origin"), Key::O);
	ED_SHORTCUT("spatial_editor/focus_selection", TTR("Focus Selection"), Key::F);
	ED_SHORTCUT("spatial_editor/align_transform_with_view", TTR("Align Transform with View"), KeyModifierMask::ALT + KeyModifierMask::CMD_OR_CTRL + Key::M);
	ED_SHORTCUT("spatial_editor/align_rotation_with_view", TTR("Align Rotation with View"), KeyModifierMask::ALT + KeyModifierMask::CMD_OR_CTRL + Key::F);
	ED_SHORTCUT("spatial_editor/freelook_toggle", TTR("Toggle Freelook"), KeyModifierMask::SHIFT + Key::F);
	ED_SHORTCUT("spatial_editor/decrease_fov", TTR("Decrease Field of View"), KeyModifierMask::CMD_OR_CTRL + Key::EQUAL); /
	ED_SHORTCUT("spatial_editor/increase_fov", TTR("Increase Field of View"), KeyModifierMask::CMD_OR_CTRL + Key::MINUS);
	ED_SHORTCUT("spatial_editor/reset_fov", TTR("Reset Field of View to Default"), KeyModifierMask::CMD_OR_CTRL + Key::KEY_0);
	PopupMenu *p;
	transform_menu = memnew(MenuButton);