You are just playing with language.
Just because something is an identical copy of something else, doesn't mean they are the same thing. For instance, you could make an identical copy of yourself and then have a conversation with yourself. That doesn't mean that, subjectively, you are both people, even if both thinks they are the original. It just means you have a similar composition and history.
The issue of identity gets murky when you start talking about replacing parts of a thing. In this case, 'thingness' becomes primarily a matter of continuity. But there is no continuity in your scenario, only similarity between two different things.
And Nietzsche was being metaphorical. Eternal return, was about embracing the life you have to such an extent that you would 'choose' to live every moment over again, forever. It was an affirmation of life.