I'm using a MySQL database to handle my member signups/logins... so was wondering, how do I make sure there aren't any duplicate signups?

at the initial signup screen, I'm asking for first name, last name(I plan on using these as the username) password and email. Also, I have another column(ID) as my primary key.

So, suppose I add address fields in here, how do I check to make sure a member doesn't register twice?