Preventing duplicate entries in mySQL database with PHP

I have a contact form (Contact Form 7) that submits all fields into a mySQL database (via the Contact Form 7 to Database Plugin), everything is working fine with that. I want it to however, throw the sender an error message if the e-mail address field has already been submitted. (And of course not submit the data).

What’s the easiest most painless way to do this?

My database’s name is wordpress

The table’s name is wpcf7dbplugin_submits