This happened with 2 different 123-reg accounts, so it's very possible it's happened to a lot of other people! And I'm sure 123-reg are having quite a headache dealing with the fall out of what was probably a tiny little accounting blip.
So if you have domains registered with 123-reg, and you've paid an auto-renewal invoice lately, just check that you only paid once / were invoiced once. In our case we received an invoice each time we were charged with a different invoice number, and in some cases domains were grouped together and in others they were charged for individually.
So just a heads up!
We've sent a report ticket to 123-reg and are hoping to receive our refunds shortly.