How can I apply ajax in php so that the email once inserted in the database should not insert in dat

i have a registeration form. and applied validation for all the fields(name,last name,email,state ).and they all are even getting inserted in the database.now I want that if email once inserted into the databse should not be inserted again in the database using ajax..
plz help me out of dis as soon as possible. and only using ajax I want to do this