Very occasionally, members who have been accepted try and re-register. Their application sits in the public area of the website awaiting approval. If approval is given they become members of both the public and private parts of the website (rather than just the private area). Recently I have been rejecting these duplicate applications but it has a knock-on effect within the private area. 99% of my members have opted-in to receive emails, but when I reject these duplicate applications their opt-in changes to opt-out of receiving emails. I then have the issue of getting them to update their profile, which is easier said than done.
I do not know why they make a second application, but I suspect that it is something to do with not being able to login (and/or forgetting their password). But this is certainly a system bug and has happened to at least 3 of my members (unless it was designed this way to happen). I also have 2 historic members that are registered twice (once in each of the public and private areas), but I suspect that if I remove them from the public area their email opt-in will change to opt-out in the private area.