Our software allows you to have more than one account with the same email user name and password. This may have been a mistake - certainly it confuses users.
We're working towards fixing this. The first step has been to add a Multiple Accounts button on the My Details page, though this only displays if you do have duplicate accounts. You can then Switch to one of the other accounts.
We will see how users want us to extend this. That might be to delete old accounts or transfer data between accounts.