### 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)

break;
				case 2:
					origin_color = get_theme_color(SNAME("axis_z_color"), EditorStringName(Editor));
					break;
				default:
					origin_color = Color();
					break;
			}
			grid_enable[i] = false;
			grid_visible[i] = false;
			origin_colors.set(x, origin_color);
			origin_colors.set(x + 1, origin_color);
			origin_colors.set(x + 2, origin_color);
			origin_colors.set(x + 3, origin_color);
			origin_colors.set(x + 4, origin_color);
			origin_colors.set(x + 5, origin_color);
			/
			/
			/
			origin_points.set(x, axis * 1048576);
			origin_points.set(x + 1, axis * 1024);
			origin_points.set(x + 2, axis * 1024);
			origin_points.set(x + 3, axis * -1024);
			origin_points.set(x + 4, axis * -1024);
			origin_points.set(x + 5, axis * -1048576);
			x += 6;
		}
		Ref<Shader> grid_shader = memnew(Shader);
		grid_shader->set_code(R"(
// 3D editor grid shader.