### 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)

}
				surftool->set_material(mat);
				surftool->commit(scale_gizmo[i]);
			}
			/
			{
				Ref<SurfaceTool> surftool = memnew(SurfaceTool);
				surftool->begin(Mesh::PRIMITIVE_TRIANGLES);
				Vector3 vec = ivec2 - ivec3;
				Vector3 plane[4] = {
					vec * GIZMO_PLANE_DST,
					vec * GIZMO_PLANE_DST + ivec2 * GIZMO_PLANE_SIZE,
					vec * (GIZMO_PLANE_DST + GIZMO_PLANE_SIZE),
					vec * GIZMO_PLANE_DST - ivec3 * GIZMO_PLANE_SIZE
				};
				Basis ma(ivec, Math_PI / 2);
				Vector3 points[4] = {
					ma.xform(plane[0]),
					ma.xform(plane[1]),
					ma.xform(plane[2]),
					ma.xform(plane[3]),
				};
				surftool->add_vertex(points[0]);
				surftool->add_vertex(points[1]);
				surftool->add_vertex(points[2]);
				surftool->add_vertex(points[0]);
				surftool->add_vertex(points[2]);
				surftool->add_vertex(points[3]);
				Ref<StandardMaterial3D> plane_mat = memnew(StandardMaterial3D);
				plane_mat->set_shading_mode(StandardMaterial3D::SHADING_MODE_UNSHADED);