MySQL error - #1062 - Duplicate entry ' ' for key 2

I'm trying to insert a huge list of users to a MySQL database but everytime I try I get the error:

#1062 - Duplicate entry '' for key 2

It gives me this because the 2nd column is blank on quite a lot of the entries, so after it's inserted one blank entry in column 2, it won't let me add another. However, when I added most of the list yesterday I didn't get this error once even though a lot of the entries I added yesterday have a blank cell in column 2 as well. Whats going on?

This is the sql code to insert 1 entry. The rest follow the same format:

INSERT INTO users
  (`id`,`title`,`firstname`,`lastname`,`company`,`address`,`city`,`county`
   ,`postcode`,`phone`,`mobile`,`category`,`email`,`password`,`userlevel`) 
VALUES     
  ('','','John','Doe','company','Streeet','city','county'
  ,'postcode','phone','','category','emial@email.co.uk','','');