Session:transient error

My new site using ci312, php 533 and mysql5083 has been running perfectly (thanks to ci developers for a great product) for over 3 months. However, about once a week (max) a session error occurs as shown below. I don't understand why it is happening. The table has the correct indices
PRIMARY KEY (`id`), UNIQUE INDEX `ci_sessions_id_ip` (`id`, `ip_address`), INDEX `ci_sessions_timestamp` (`timestamp`).
Apart from this there are no other log entries. Any suggestions?
Many thanks,
Bill

ERROR - 03-17 22:08:44 --> Query error: Duplicate entry 'd5c0884d53d8ddd3d0a077ab6f7827491619506a' for key 1 - Invalid query: INSERT INTO `ci_sessions` (`id`, `ip_address`, `timestamp`, `data`) VALUES ('d5c0884d53d8ddd3d0a077ab6f7827491619506a', '93.107.38.150', 1489788524, '__ci_last_regenerate|i:1489788521;|a:2:{i:0;s:1:\"2\";i:1;s:4:\"2/40\";}pretrolley_page|s:4:\"2413\";previous_page|s:4:\"2413\";')
ERROR - 03-17 22:08:44 --> Severity: Warning --> Unknown: Failed to write session data (user). Please verify that the current setting of session.save_path is correct (/usr/local/pem/vhosts/141487/tmp/) Unknown 0