If the data is public anyway (eg usernames on forums) then you acn check it before submitting - it's availanle to anyone anyway.
If the data isn't public and sensitive...
You know, if the email address is the right one, then the registration form would be sent (and delivered, most of the time) to it, but if it's not - then not. So for the emails there is no reason to disclose the existence of the account with such email at all.
But you should occasionly check you mail bounces and attempts to register with an existing addresses.
Comments
If the data is public anyway (eg usernames on forums) then you acn check it before submitting - it's availanle to anyone anyway.
If the data isn't public and sensitive...
You know, if the email address is the right one, then the registration form would be sent (and delivered, most of the time) to it, but if it's not - then not. So for the emails there is no reason to disclose the existence of the account with such email at all.
But you should occasionly check you mail bounces and attempts to register with an existing addresses.