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Then the banks have solved the double-spending problem, so in effect there isn't one.

I was asking the question because if double-spending has been solved, then why do we need crypto/blockchain?

Sure, if double-spending has been solved for you, you don't need a blockchain. It hasn't been solved for everyone yet though. A great book to learn about this is: Check Your Financial Privilege (https://www.amazon.com/Check-Your-Financial-Privilege-Gladst...)

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