Tap on both and see if one still says Pending while the other doesn’t. That will be refunded after a week, but if you need the money urgently you can tap ‘Speak to us’ (or similar) to send Monzo a message about it, and they can reverse it (they may want a screenshot of your bill to be sure).
Similar happened with Sky recently and Now was mentioned on the thread too - Sky Bill - Charged Twice