I agree that it’d be best if it told the user the email is being used, and to login instead of signing up again. That said, it doesn’t prevent an existing user from signing up with a second account if he uses a different email address then the first time by mistake.
I think it’s important to have a message reminding the existing user to login.
I mentioned selling only to logged in users because then you eliminate the problem of selling to an existing user while he’s logged out. I know it adds the step of registration first, and it’d seem it will cause loses, but you could make that free registration attractive with a freebie, and not hard asking only the bare minimum of fields (similar to an email opt-in).
But that’s a separate issue from the one where the existing subscription for level 1 is ended when he then purchases level 1 with a one-time payment. We were talking about existing users, which is when that issue would happen.
The reason why the logged in/out topic came up is because I’m suggesting selling the ccap individually, without the level. That works for the logged in user, but isn’t possible for a logged out user, so it requires the level to be added to the form.
When selling individual ccaps, I think it’s fine to have a conditional that doesn’t add a level to the form when the user is logged in, so it doesn’t affect existing subscriptions, but adds the level when the person is logged out so the ccap can be sold to him. Not perfect, but works.