Duplicate entry error in db

[Boofo]

Can anyone tell me why this is happening all of a sudden when I new user tried to register? I am using vb 4.1.3, if that helps.

Invalid SQL:
INSERT INTO userfield
        (field7, field8, field5, userid)
VALUES
        ('Off', 'http://www.google.com', 'x.xxxxxx@xxxxxxx.net', 19);

MySQL Error   : Duplicate entry '19' for key 1
Error Number  : 1062

NOTE: I x'd out the user's email address.