2 questions

1. How can I prevent multiple entries in a form securely?

For example a user with two different emails fills out the form twice. I want to be able to block a user from entering data a second time regardless of how many zoho accounts/emails they might have. Can this be done?

2. I want to make it so that my form can only be seen by user who have a zoho/gmail/yahoo/etc. account without making it private. Can this be done?

Thanks in advance,
dgodfather