One submit per email address

One more question guys. I have registration form and I need to disable multiple registration with only one email address. So lets say - contact comes to register itself at my register form and he will make the first time registration - registration is successed.



But second day, the contact, with the same email address will forget that he already is registered so he will fill up the form with the same email address - and this is the right moment to let the form warn the client “Hey, we already have this email address, you can not register itself again”.



Is it possible?

One more question guys. I have registration form and I need to disable multiple registration with only one email address. So lets say - contact comes to register itself at my register form and he will make the first time registration - registration is successed.

But second day, the contact, with the same email address will forget that he already is registered so he will fill up the form with the same email address - and this is the right moment to let the form warn the client “Hey, we already have this email address, you can not register itself again”.

Is it possible?