### 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)

case TRANSFORM_VIEW:
					/
					global_axis = _get_camera_normal();
					break;
				case TRANSFORM_X_AXIS:
					local_axis = Vector3(1, 0, 0);
					break;
				case TRANSFORM_Y_AXIS:
					local_axis = Vector3(0, 1, 0);
					break;
				case TRANSFORM_Z_AXIS:
					local_axis = Vector3(0, 0, 1);
					break;
				case TRANSFORM_YZ:
				case TRANSFORM_XZ:
				case TRANSFORM_XY:
					break;
			}
			if (_edit.plane != TRANSFORM_VIEW) {
				global_axis = spatial_editor->get_gizmo_transform().basis.xform(local_axis).normalized();
			}
			Vector3 intersection;
			if (!plane.intersects_ray(ray_pos, ray, &intersection)) {
				break;
			}
			Vector3 click;
			if (!plane.intersects_ray(_edit.click_ray_pos, _edit.click_ray, &click)) {
				break;
			}
			static const float orthogonal_threshold = Math::cos(Math::deg_to_rad(87.0f));