What would cause a duplicate session key?

Hi All,

I occationally recieve the following error resulting in failure of several pieces of code later in the pipeline.

PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 'MTQ4MjgxNDg3ODE2Mjk5NDM5NDU-MTQ4MjgxNDg3ODQ3NjcxODAwMg' for key 'PRIMARY': UPDATE {sessions} SET sid=:db_update_placeholder_0, ssid=:db_update_placeholder_1 WHERE (ssid = :db_condition_placeholder_0) ; Array ( [:db_update_placeholder_0] => MTQ4MjgxNDg3ODE2Mjk5NDM5NDU [:db_update_placeholder_1] => MTQ4MjgxNDg3ODQ3NjcxODAwMg [:db_condition_placeholder_0] => MTQ4MjgxMjcxMjkxOTQwMjEyOQ ) in drupal_session_regenerate() (line 393 of /home/makemeupbeauty/public_html/includes/session.inc).

The error occurs on the user login/register submit part.

I suspect that drupal_session_start is causing the issue but I don't understand sessions well enough to debug this.

Anyone know about session that can enlighten me to how to track this issue down?

Thanks