### 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)

message = p_message;
	message_time = p_time;
}
void Node3DEditorPlugin::edited_scene_changed() {
	for (uint32_t i = 0; i < Node3DEditor::VIEWPORTS_COUNT; i++) {
		Node3DEditorViewport *viewport = Node3DEditor::get_singleton()->get_editor_viewport(i);
		if (viewport->is_visible()) {
			viewport->notification(Control::NOTIFICATION_VISIBILITY_CHANGED);
		}
	}
}
void Node3DEditorViewport::_project_settings_changed() {
	
	int shadowmap_size = GLOBAL_GET("rendering/lights_and_shadows/positional_shadow/atlas_size");
	bool shadowmap_16_bits = GLOBAL_GET("rendering/lights_and_shadows/positional_shadow/atlas_16_bits");
	int atlas_q0 = GLOBAL_GET("rendering/lights_and_shadows/positional_shadow/atlas_quadrant_0_subdiv");
	int atlas_q1 = GLOBAL_GET("rendering/lights_and_shadows/positional_shadow/atlas_quadrant_1_subdiv");
	int atlas_q2 = GLOBAL_GET("rendering/lights_and_shadows/positional_shadow/atlas_quadrant_2_subdiv");
	int atlas_q3 = GLOBAL_GET("rendering/lights_and_shadows/positional_shadow/atlas_quadrant_3_subdiv");
	viewport->set_positional_shadow_atlas_size(shadowmap_size);
	viewport->set_positional_shadow_atlas_16_bits(shadowmap_16_bits);
	viewport->set_positional_shadow_atlas_quadrant_subdiv(0, Viewport::PositionalShadowAtlasQuadrantSubdiv(atlas_q0));
	viewport->set_positional_shadow_atlas_quadrant_subdiv(1, Viewport::PositionalShadowAtlasQuadrantSubdiv(atlas_q1));