### 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)

environ_energy->set_min(0);
		environ_energy->set_step(0.05);
		environ_energy->connect("value_changed", callable_mp(this, &Node3DEditor::_preview_settings_changed).unbind(1));
		environ_vb->add_margin_child(TTR("Sky Energy"), environ_energy);
		HBoxContainer *fx_vb = memnew(HBoxContainer);
		fx_vb->set_h_size_flags(SIZE_EXPAND_FILL);
		environ_ao_button = memnew(Button);
		environ_ao_button->set_text(TTR("AO"));
		environ_ao_button->set_toggle_mode(true);
		environ_ao_button->connect("pressed", callable_mp(this, &Node3DEditor::_preview_settings_changed), CONNECT_DEFERRED);
		fx_vb->add_child(environ_ao_button);
		environ_glow_button = memnew(Button);
		environ_glow_button->set_text(TTR("Glow"));
		environ_glow_button->set_toggle_mode(true);
		environ_glow_button->connect("pressed", callable_mp(this, &Node3DEditor::_preview_settings_changed), CONNECT_DEFERRED);
		fx_vb->add_child(environ_glow_button);
		environ_tonemap_button = memnew(Button);
		environ_tonemap_button->set_text(TTR("Tonemap"));
		environ_tonemap_button->set_toggle_mode(true);
		environ_tonemap_button->connect("pressed", callable_mp(this, &Node3DEditor::_preview_settings_changed), CONNECT_DEFERRED);
		fx_vb->add_child(environ_tonemap_button);
		environ_gi_button = memnew(Button);
		environ_gi_button->set_text(TTR("GI"));
		environ_gi_button->set_toggle_mode(true);
		environ_gi_button->connect("pressed", callable_mp(this, &Node3DEditor::_preview_settings_changed), CONNECT_DEFERRED);
		fx_vb->add_child(environ_gi_button);
		environ_vb->add_margin_child(TTR("Post Process"), fx_vb);
		environ_add_to_scene = memnew(Button);
		environ_add_to_scene->set_text(TTR("Add Environment to Scene"));
		environ_add_to_scene->set_tooltip_text(TTR("Adds a WorldEnvironment node matching the preview environment settings to the current scene.\nHold Shift while clicking to also add the preview sun to the current scene."));