I’ve had this happen a couple of times in Tesco.
Of the two transactions, one will say ‘Pending’ at the bottom. This one will automatically drop off after a week or so.
If you need to contact COps to get a reversal sooner, you need to make sure you apply for the reversal on the ‘Pending’ transaction. If Tesco un-reverse it (unlikely to happen) then the reason that you have to contact Tesco is because it is their mistake, not Monzo’s. If that were to happen, they’re the ones who’ve taken your money twice. Not Monzo.