How do I make sure that duplicate data isn't stored in a table?

In other words, if record 1 has the email address "", I want to make sure that no other records that are created can have the email address

Right now, I'm solving this problem through PHP by checking if that email address exists before inserting it. But isn't there a way to do this through mysql? I though there was a certain way to set up a table so that duplicate data couldn't be inserted.