I've been doing their shops for about a month.
Yesterday was the first time I got a second confirmation like you describe.
I had two shops, but got a reminder for only one. But it didn't say which one. It seems very confusing.
I think it is supposed to serve as a polite reminder if you are nearing your deadline. In my case, one of the shops was due that day and had to be reported by 9pm. The other shop wasn't due for three more days. The email was sent to me just after 2pm on the day the shop was due.