### 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)

col_axis = i;
				}
			}
		}
		bool is_plane_translate = false;
		
		if (col_axis == -1) {
			col_d = 1e20;
			for (int i = 0; i < 3; i++) {
				Vector3 ivec2 = gt.basis.get_column((i + 1) % 3).normalized();
				Vector3 ivec3 = gt.basis.get_column((i + 2) % 3).normalized();
				
				
				const Vector3 grabber_pos = gt.origin + (ivec2 + ivec3) * gizmo_scale * (GIZMO_PLANE_SIZE + GIZMO_PLANE_DST * 0.6667);
				Vector3 r;
				Plane plane(gt.basis.get_column(i).normalized(), gt.origin);
				if (plane.intersects_ray(ray_pos, ray, &r)) {
					const real_t dist = r.distance_to(grabber_pos);
					
					
					if (dist < (gizmo_scale * GIZMO_PLANE_SIZE * 1.5)) {
						const real_t d = ray_pos.distance_to(r);
						if (d < col_d) {
							col_d = d;
							col_axis = i;
							is_plane_translate = true;
						}
					}
				}
			}