### 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)

settings_dialog->set_title(TTR("Viewport Settings"));
	add_child(settings_dialog);
	settings_vbc = memnew(VBoxContainer);
	settings_vbc->set_custom_minimum_size(Size2(200, 0) * EDSCALE);
	settings_dialog->add_child(settings_vbc);
	settings_fov = memnew(SpinBox);
	settings_fov->set_max(MAX_FOV);
	settings_fov->set_min(MIN_FOV);
	settings_fov->set_step(0.1);
	settings_fov->set_value(EDITOR_GET("editors/3d/default_fov"));
	settings_fov->set_select_all_on_focus(true);
	settings_vbc->add_margin_child(TTR("Perspective FOV (deg.):"), settings_fov);
	settings_znear = memnew(SpinBox);
	settings_znear->set_max(MAX_Z);
	settings_znear->set_min(MIN_Z);
	settings_znear->set_step(0.01);
	settings_znear->set_value(EDITOR_GET("editors/3d/default_z_near"));
	settings_znear->set_select_all_on_focus(true);
	settings_vbc->add_margin_child(TTR("View Z-Near:"), settings_znear);
	settings_zfar = memnew(SpinBox);
	settings_zfar->set_max(MAX_Z);
	settings_zfar->set_min(MIN_Z);
	settings_zfar->set_step(0.1);
	settings_zfar->set_value(EDITOR_GET("editors/3d/default_z_far"));
	settings_zfar->set_select_all_on_focus(true);
	settings_vbc->add_margin_child(TTR("View Z-Far:"), settings_zfar);
	for (uint32_t i = 0; i < VIEWPORTS_COUNT; ++i) {
		settings_dialog->connect("confirmed", callable_mp(viewports[i], &Node3DEditorViewport::_view_settings_confirmed).bind(0.0));
	}
	/