Doesn't the bank ask the account number twice precisely to avoid this exact situation?

Yeah but sometimes you think the account number you entered first is the correct one, just a normal human error. Mostly occurs with new account numbers.

But how its possible?

If account number is incorrect the beneficiary name wouldn't match.

Account number and account holder name both should match.

I don't know exactly whats the case with OP here but lets say you got a number of beneficiaries attached to your account , I use Internet banking and I just have to select the beneficiary from the list to complete the transaction once I mentioned the amount and sometimes a beneficiary with whom you just had few transaction get selected , just normal human error.

 You need to add account number and account holder's name for the first time when you add a beneficiary to the account after that just hit the radio button and thats it, at least this is how it works in the internet banking software of bank I use.