An unlikely scenario but one I've always wondered about:
Let's say that a murder was committed. There is a pair of identical twins. Both twins had means, motive, and opportunity for the murder.
It has been proven beyond reasonable doubt that one of them committed the crime. A witness actually saw one of the twins stab someone, to death but he can't tell which one. The knife has been found, and has the twins' DNA on it. Of course, their DNA is identical, so it is evidence that one of them did it, but tells us nothing about which one. Neither twin has been able to establish an alibi.
Am I correct in assuming that a conviction is impossible, because either twin could argue that the other did it? What if one twin established an alibi, would that be sufficient to convict the other?