I have had the same issue with NowTV and being charged twice. I contacted support and they refunded one of the payments immediately and said it was a known issue.
I am quite disappointed with Monzo, normally when something like this happens, they get ahead of the issue, inform those involved and explain what has happened. Because of this I immediately contacted NowTV and accused them of making the mistake.
The only reason Monzo have continued to succeed despite their repeated outages and issues is because of their outstanding transparency and the way they deal with incidents. I think they have let themselves down on this occasion, even if it is not directly their fault, I expect better communication and don’t expect to have to ask for a refund.