Prevent duplicate accounts
A way to prevent (or at least limit the number of) users creating duplicate accounts by not allowing a user to use the same email address to create a new account, but then have them prompted to "Change Password" if they try to create a new account using the same email address.
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Marilyn Rackley commented
A user trying to create an account with an email address already in the database is probably already registered and should not be creating another account.