### 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 plane(gt.basis.get_column(i).normalized(), gt.origin);
				Vector3 r;
				if (!plane.intersects_ray(ray_pos, ray, &r)) {
					continue;
				}
				const real_t dist = r.distance_to(gt.origin);
				const Vector3 r_dir = (r - gt.origin).normalized();
				if (_get_camera_normal().dot(r_dir) <= 0.005) {
					if (dist > gizmo_scale * (GIZMO_CIRCLE_SIZE - GIZMO_RING_HALF_WIDTH) && dist < gizmo_scale * (GIZMO_CIRCLE_SIZE + GIZMO_RING_HALF_WIDTH)) {
						const real_t d = ray_pos.distance_to(r);
						if (d < col_d) {
							col_d = d;
							col_axis = i;
						}
					}
				}
			}
		}
		if (col_axis != -1) {
			if (p_highlight_only) {
				spatial_editor->select_gizmo_highlight_axis(col_axis + 3);
			} else {
				
				_edit.mode = TRANSFORM_ROTATE;
				_compute_edit(p_screenpos);
				_edit.plane = TransformPlane(TRANSFORM_X_AXIS + col_axis);
			}
			return true;
		}
	}