### 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)

plane_mv = true;
					break;
				case TRANSFORM_XY:
					motion_mask = spatial_editor->get_gizmo_transform().basis.get_column(0).normalized() + spatial_editor->get_gizmo_transform().basis.get_column(1).normalized();
					plane = Plane(spatial_editor->get_gizmo_transform().basis.get_column(2).normalized(), _edit.center);
					plane_mv = true;
					break;
			}
			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;
			}
			Vector3 motion = intersection - click;
			if (_edit.plane != TRANSFORM_VIEW) {
				if (!plane_mv) {
					motion = motion_mask.dot(motion) * motion_mask;
				} else {
					/
					if (p_shift) {
						motion = motion_mask.dot(motion) * motion_mask;
					}
				}
			} else {
				const real_t center_click_dist = click.distance_to(_edit.center);
				const real_t center_inters_dist = intersection.distance_to(_edit.center);
				if (center_click_dist == 0) {