It is hard to know for sure, as only Monzo will know, and if you have any doubts, by all means contact them using the in app chat. They are usually extremely fast as well, so please go ahead.
However, by and large that’s unlikely. They way card payments work is as follows (and I’m hugely simplifying this):
- The merchant contacts your bank and says “This person wants to pay £18.99. Are you alright with that?”
- Bank responds. “Sure thing. I’ll reserve £18.99 from their account balance, you have until next week to collect it.”. No money actually changes hands here: It’s just blocked for you.
- Some time later (maybe the same day, but it can be up to a week later) the merchant goes back to your bank and says “Remember I asked for £18.99 recently? I’d like to have these now.”
- Bank says “There you go” and sends the money.
What has most likely happened here is that the merchant, rather than collecting the amount it had originally reserved, made a completely new transaction. The “reservation” will eventually (usually after 7 days) expire by itself.
As others have said: This practice seems to be common in the US. This has nothing to do with Monzo, but with the merchant. (If you were doing the same with, e.g. your Barclays debit card, the same thing would happen. But because you don’t get instant notifications you wouldn’t notice, as the original “reservation” would never show up in your statement.)
If you have any concerns, though, please go ahead and contact Monzo directly through the app. They can look into it, and cancel the “reservation” before it expires naturally.
(If you are interested: The “reservation” is called “authorisation” and the “collection” “presentment”.