Well, I duly sent a grumpy email to this address yesterday, and, surprise, surprise, I received the following reply more or less by return, which confirmed my suspicions as suggested up thread that duplicate registrations are being weeded out!
Your registration has been approved. The reject email was for the duplicate registration which you may have accidentally completed. Please do not worry as it would not affect your status in the registration.
You will be sent an appointment details once your vaccine has been allocated by the Ministry of Public Health. At this point, the majority of registrants' info has been sent to each province and to the Ministry of Public Health, and more continue to be forwarded.
The waiting time could vary from group to group and province to province. Normally those in the high priority group are notified within a few weeks.
Please note that each province's vaccination scheme is progressing at a different pace, owing to the different capacity of local medical facilities and the fact that the system prioritises those in the vulnerable groups such as persons aged 60 or over and those with underlying medical conditions first.