Tried to access a notification message (post approval request) as a board administrator, and got the following error message:

General Error

SQL ERROR [ mysqli ]

Duplicate entry 'post_in_queue' for key 'type' [1062]

SQL

INSERT INTO op8_notification_types (notification_type_name, notification_type_enabled) VALUES ('post_in_queue', 1)

BACKTRACE

FILE: (not given by php)
LINE: (not given by php)
CALL: msg_handler()

FILE: [ROOT]/phpbb/db/driver/driver.php
LINE: 999
CALL: trigger_error()

FILE: [ROOT]/phpbb/db/driver/mysqli.php
LINE: 193
CALL: phpbb\db\driver\driver->sql_error()

FILE: [ROOT]/phpbb/db/driver/factory.php
LINE: 329
CALL: phpbb\db\driver\mysqli->sql_query()

FILE: [ROOT]/phpbb/notification/manager.php
LINE: 933
CALL: phpbb\db\driver\factory->sql_query()

FILE: [ROOT]/phpbb/notification/manager.php
LINE: 167
CALL: phpbb\notification\manager->get_notification_type_id()

FILE: [ROOT]/includes/mcp/mcp_queue.php
LINE: 178
CALL: phpbb\notification\manager->mark_notifications()

FILE: [ROOT]/includes/functions_module.php
LINE: 676
CALL: mcp_queue->main()

FILE: [ROOT]/mcp.php
LINE: 312
CALL: p_master->load_active()
p_master->load_active()

I was able to access it the first time, however I did something else for a minute (I think it was checking the user's credentials), and then came back and tried to access the notification again, when the error occurred.

May be related to this issue:

https://tracker.phpbb.com/browse/PHPBB3-15105