### 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)

}
	if (previewing) {
		Size2 ss = Size2(GLOBAL_GET("display/window/size/viewport_width"), GLOBAL_GET("display/window/size/viewport_height"));
		float aspect = ss.aspect();
		Size2 s = get_size();
		Rect2 draw_rect;
		switch (previewing->get_keep_aspect_mode()) {
			case Camera3D::KEEP_WIDTH: {
				draw_rect.size = Size2(s.width, s.width / aspect);
				draw_rect.position.x = 0;
				draw_rect.position.y = (s.height - draw_rect.size.y) * 0.5;
			} break;
			case Camera3D::KEEP_HEIGHT: {
				draw_rect.size = Size2(s.height * aspect, s.height);
				draw_rect.position.y = 0;
				draw_rect.position.x = (s.width - draw_rect.size.x) * 0.5;
			} break;
		}
		draw_rect = Rect2(Vector2(), s).intersection(draw_rect);
		surface->draw_rect(draw_rect, Color(0.6, 0.6, 0.1, 0.5), false, Math::round(2 * EDSCALE));
	} else {
		if (zoom_indicator_delay > 0.0) {
			if (is_freelook_active()) {
				/
				real_t min_speed = MAX(camera->get_near() * 4, ZOOM_FREELOOK_MIN);
				real_t max_speed = MIN(camera->get_far() / 4, ZOOM_FREELOOK_MAX);
				real_t scale_length = (max_speed - min_speed);
				if (!Math::is_zero_approx(scale_length)) {
					real_t logscale_t = 1.0 - Math::log1p(freelook_speed - min_speed) / Math::log1p(scale_length);
					/