Error - Duplicate entry

When a visitor/guest (i.e. a non-registered user) tries to post something they get the following error.

user error: Duplicate entry '' for key 2
query: INSERT INTO users (created, changed, uid) VALUES ('1120635816', '1120635816', '7') in //includes/database.mysql.inc on line 66.

warning: Cannot modify header information - headers already sent by (output started at /includes/common.inc:384) in /includes/common.inc on line 192.

The content they were submitting gets added, but the error appears each time they try post something new.

Also the uid is used.... thus when a non-registered users make a post the uid for the next user to register is increased.
For instance, I made a 5 posts as a non-registered user before the posts the next available uid was 4, however, after the posts the next available uid was 9.