You have been charged twice. The money is gone from your account. Presumably one of those charges will be (automatically) refunded but John Lewis could have expedited that and chose not to, because they didn’t care and they normally get away with it.
For a lot of people losing that much money unexpectedly could cause serious problems like rent or mortgage being unable to be paid. And in legacy banks that’d happen without any warning. Suddenly they’d be in financial mire through no fault of their own. Monzo pins the blame firmly on John Lewis, as it should do.