I liked the finale but there are plot holes.   Its pretty clear that the Peter of this world died and Walter snatched the Peter from the other world to replace him.   Now think of that.  If there are two of everyone, and one dies as a child and the other lives to grow up, and presumably have children.  Doesn't that change one of the worlds?   

  They established that one can die on one world, but your doppleganger can go on living in the other.   Just look at last week's episode where one of the twins died on this world, but both died on the other.

So the worlds can not be the same.  In fact, if one person died say 1000 years ago, but their copy didn't die on the other world, then their offspring wouldn't exist either.   This would populate one world with different people than on the other.   A copy of one Earth can exist, but not one with a duplicate of everyone, for the reasons  I gave above, unless one dying on one world also kills  them in the other.