

It means the following in your example:
Suppose I am one of twins, but I don't know which twin I am. But I know that twin 1 will be hit in face in next 1 minute. In this case I am in the situation of so called indexical uncertainty and my best guess is that I have 50 per cent probability of being twin 1 and thus 50 per cent probability being hit in the future. After I will be hit, I will know with 100 per cent probability that I am twin 1.
In general there is problem with the idea of copies, as it mixes several different ideas and it results into paradoxes. The idea of copies should be broken on several ideas:
1) "past copies" - that is the copies of me which was made before now moment (twins are best example of them),
2) "mirror copies" which are exact my copies now
3) "future copies" - which will be made from me in future.
Thanks for the clarification. However when you say:
Suppose I am one of twins, but I don't know which twin I am. But I know that twin 1 will be hit in face in next 1 minute. In this case I am in the situation of so called indexical uncertainty and my best guess is that I have 50 per cent probability of being twin 1 and thus 50 per cent probability being hit in the future. After I will be hit, I will know with 100 per cent probability that I am twin 1.
This is really not unique to twins. Suppose I choose two strangers and tell them that I will flip a coi...