Even simpler analogy to work from: you have a file which has your list of things you bought and sold. The file is called 'transactions.txt' You make a copy of that file, calling it 'transactions_2.txt' and then start appending what you bought and sold on that. But then, you are also still appending to the old file what you bought and sold.

The Bitcoin/BCH thing is like this, but on a greater scale with potential risks around it (BCH had to ensure there were no replay attacks on the Bitcoin blockchain for example.) For example, Trezor recommends (and has to deliver your Bitcoin to a Bcash-specific address: https://trezor.io/claim-bch/.