I know that there are some things, when someone tries to use ATM/else when phone location doesn’t match and similar ideas (or maybe already in place).
I’d like something from this area, but focused on multiple transactions.
Recently, it happened that one of my transactions went through, but errored on merchant’s page. I assumed it didn’t go through, because it was payment that failed. I made another purchase and I was charged twice. I didn’t notice it until 5 days later, because it was Barclays with rubbish phone app and no notifications at all. This is a lot less likely to happen with Monzo, but knowing me as a user… situations like this can fly under my radar even with Monzo. 
Monzo is sending many notifications, and I’m slowly getting worried that I’ll dismiss them brainlessly, without fully realising what they mean. I’d like ‘suspicious activity’ = on/off setting in Monzo.
With this setting enabled, Monzo would send me separate notification (with many danger noodle emojis and exclamation marks so I pay attention), for example when:
- 2, 3 or more purchases are made within short time period to the same merchant.
- 2 or more purchases for exactly same amount are made on the same day or at least within hours of each other.
It’s cognitive bias in a way, I can’t tell if I’m seeing same notification again (but I forgot to dismiss it earlier), or is it new notification. And because I rarely suspect the worst, I might not see duplicated or fraudulent transactions until I open up monzo app and put my brain to work.
I can see how this might be false-positive for people buying coffee in the morning and in the evening in the same coffee shop. Or sitting at the airport and buying 1 coffee/hour for 3 hours. However, setting can be disabled and it’s only outcome is warning notification from Monzo - so nothing bad could happen. 
It could also lead to slightly more advanced feature. System could observe and attempt to detect a bit too many contactless payments within certain time frame (card was stolen). Put it as a warning to user via app. In critical situations, call from support about extremely suspicious activity.
This can be confusing and inconvenient to say the least and we’re working on improving our processes to prevent this happening where possible!