### 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_state = memnew(Label);
		sun_environ_hb->add_child(sun_state);
		sun_state->set_horizontal_alignment(HORIZONTAL_ALIGNMENT_CENTER);
		sun_state->set_vertical_alignment(VERTICAL_ALIGNMENT_CENTER);
		sun_state->set_h_size_flags(SIZE_EXPAND_FILL);
		VSeparator *sc = memnew(VSeparator);
		sc->set_custom_minimum_size(Size2(50 * EDSCALE, 0));
		sc->set_v_size_flags(SIZE_EXPAND_FILL);
		sun_environ_hb->add_child(sc);
		environ_vb = memnew(VBoxContainer);
		sun_environ_hb->add_child(environ_vb);
		environ_vb->set_custom_minimum_size(Size2(200 * EDSCALE, 0));
		environ_vb->hide();
		environ_title = memnew(Label);
		environ_title->set_theme_type_variation("HeaderSmall");
		environ_vb->add_child(environ_title);
		environ_title->set_text(TTR("Preview Environment"));
		environ_title->set_horizontal_alignment(HORIZONTAL_ALIGNMENT_CENTER);
		environ_sky_color = memnew(ColorPickerButton);
		environ_sky_color->set_edit_alpha(false);
		environ_sky_color->connect("color_changed", callable_mp(this, &Node3DEditor::_preview_settings_changed).unbind(1));
		environ_sky_color->get_popup()->connect("about_to_popup", callable_mp(EditorNode::get_singleton(), &EditorNode::setup_color_picker).bind(environ_sky_color->get_picker()));
		environ_vb->add_margin_child(TTR("Sky Color"), environ_sky_color);
		environ_ground_color = memnew(ColorPickerButton);
		environ_ground_color->connect("color_changed", callable_mp(this, &Node3DEditor::_preview_settings_changed).unbind(1));
		environ_ground_color->set_edit_alpha(false);
		environ_ground_color->get_popup()->connect("about_to_popup", callable_mp(EditorNode::get_singleton(), &EditorNode::setup_color_picker).bind(environ_ground_color->get_picker()));
		environ_vb->add_margin_child(TTR("Ground Color"), environ_ground_color);
		environ_energy = memnew(EditorSpinSlider);
		environ_energy->set_max(8.0);