Please watch this all the way to the end https://en.wikipedia.org/wiki/The_Prestige and be very careful as you watch the final scenes.

For reference, teleportation has the same problem.

The difference between fiction and reality is that fiction doesn't have to be self-consistent. Who knows how real teleportation or mind uploading will work, even hypothetically supposing that it is possible? Maybe high fidelity brain scanning or teleportation would be destructive.

If we're talking fictional uploading, I personally prefer Accelerando (Stross) or Diaspora (Egan). I have no evidence that one is more realistic than the other (far future tech is impossible to predict) but I find them more compelling and interesting than the old "don't play god or bad things happen" narratives.