Some forms require that the email submitted in the form to be unique. If the user has already submitted the form, the form should not be submitted again (with the same email, at least)

Here is an extension module that checks the database to see that the email was not submitted before.
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Sometimes, you would want to restrict your form to a limited set of users. Only those who have a password can submit the form.

Here is an extension module to password protect your form.
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