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

Question

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','[email protected]','',''); 

Answer 1

In addition to Sabeen's answer:

The first column id is your primary key.
Don't insert '' into the primary key, but insert null instead.

INSERT INTO users   (`id`,`title`,`firstname`,`lastname`,`company`,`address`,`city`,`county`    ,`postcode`,`phone`,`mobile`,`category`,`email`,`password`,`userlevel`)  VALUES        (null,'','John','Doe','company','Streeet','city','county'   ,'postcode','phone','','category','[email protected]','',''); 

If it's an autoincrement key this will fix your problem.
If not make id an autoincrement key, and always insert null into it to trigger an autoincrement.

MySQL has a setting to autoincrement keys only on null insert or on both inserts of 0 and null. Don't count on this setting, because your code may break if you change server.
If you insert null your code will always work.

See: http://dev.mysql.com/doc/refman/5.0/en/example-auto-increment.html

Answer 2

Seems like the second column is set as a unique index. If you dont need that remove it and your errors will go away. Possibly you added the index by mistake and thats why you are seeing the errors today and werent seeing them yesterday