I would like to clarify my position: Identity is complex social adaptation and it is directly connected with hard problem of consciousness. So we can't finally solve any identity paradox on our current level of knowledge.

Indexical uncertainty in case of twins is a trick which may be used to skip identity problem. It doesn't prove that twins are identical. It just makes it not important.

So it doesn't prove that twins are the same. It may work for very different people as long as everyone don't know who is who. But it could be reasonable guide to make decisions in the situations where many my copies exist (including uploading, quantum multiverse statistic etc)

But it is not the only principle. Another one is "conservative approach" - that is we should try to preserve as much identity as possible as we don't know what is identity.