People do go back to the signup page a lot. Redirecting them when detecting an existing account from the email is enough in the most common cases. Users doing

People do go back to the signup page a lot. Redirecting them when detecting an existing account from the email is enough in the most common cases. Users doing "clever" stuff like a random email each time, will also probably understand the situation when they go write down which email for which domain they used and see another entry next to it. There will be some numbers of users that can't be caught and will create many new accounts, but if thwy use your seevice that much, it will probably stop at one point. Nothing is foolproof, but there can be many ways to mitigate the issue.