Hi, Ruben from Monzo here (again).
I just wanted to drop a note to say that this should all be fixed now.
We put a patch live on our side on the 29th June (with a couple improvements that went live on the 5th and 6th) that made most if not all duplicate transactions to stop.
Then on the 21st July we were told that a fix had been applied by WorldPay. When we looked closer into it we noticed this appears to have gone live on the 19th July. By that point our patch should’ve made the problem vanish pretty much entirely, but it’s good to know that this is also now fixed at the root.
If you see any more instances of this happening, please let our support team know and they’ll be able to help out via our usual routes.