sometimes get error Duplicate entry for key 'PRIMARY'

No, but if you want to use 'sess_match_ip', you must add a unique constraint for id, ip_address ... like explained in the manual: http://www.codeigniter.com/userguide3/li...ase-driver

it works ...
thank you ... 

Code:

PRIMARY KEY (`id`),
UNIQUE INDEX `id_ip_address` (`id`, `ip_address`),
INDEX `timestamp` (`timestamp`)
)