Why do you want to detect the difference between a user signing up with one email account twice or two different accounts? It's folly to try to detect this in this manner and you will only piss off regular users.
i understand why they do it, but i can't condone it off course.
The service should just allow it, and make sure the appropriated subscribing fee.
I can always create multiple email accounts, you know. So I don't see how spending development efforts to parse and detect + and . is effective.