You can't avoid it, username existence is easy to check by trying to register an account. The only solution against the problem you described is to use username

You can't avoid it, username existence is easy to check by trying to register an account. The only solution against the problem you described is to use usernames, which are freely chosen by the user or generated by the website automatically, not email addresses, so that people can create accounts not tied to their identity. Also, two-step form doesn't mean that you have to check username existence after the first step. But, again, this is useless as mentioned above.