No worries Angus! I have no expectations of your attention!
The user accounts are created through a purchase being made on woocommerce. Woo used to be on a subsite, but the sites were merged. The two sites shared a user table so the IDs should, at least in theory, have remained constant.
Access to the website was, before my time, controlled via membermouse. Since then - around 5 years ago - the access has been controlled by imember360 and recently replaced with memberium, both essentially identical plugins that integrate with a CRM called infusionsoft and allow access to various pages based on data from the crm.
The way these plugins work require a unique email address so it’s impossible for the same email address to be used in multiple users.
It’s an ongoing problem, but seems to be limited to a very small number of users. It’s happened to two ‘old’ users, and recently happened to a 5 days old account, so isn’t an issue for new users only.