Duplicate entry errors

I started seeing errors like this being logged on a site we recently launched that requires users to log in. What causes this to happen and how can it be prevented? Thanks in advance!

PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 'ST-7566128-ZhlhkQEWGXYpsaeQIrut-cas' for key 'PRIMARY': INSERT INTO {cas_login_data} (cas_session_id, uid) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1); Array ( [:db_insert_placeholder_0] => ST-7566128-ZhlhkQEWGXYpsaeQIrut-cas [:db_insert_placeholder_1] => 11288 ) in _cas_single_sign_out_save_token() (line 1123 of /data01/d7/sites/all/modules/contrib/cas/cas.module).