If you fork the bitcoin blockchain you have now two ledgers, each one telling you that a certain number of bitcoins really exist. Who is to say that one is right and the other is wrong? In this scenario, it's debatable how many bitcoins are in existence.
You might want to read the Bitcoin whitepaper. There is zero ambiguity in the situation you describe. That is the essence of the proof-of-work (or proof-of-stake, take your pick) solution to the Byzantine generals problem.
