For the last 2-3 releases the same issue is popping up periodically:
[INSERT INTO ost_thread_referral SET thread_id = 36764, object_id = 4, object_type = 'D', created = NOW()]
Duplicate entry '4-D-36764' for key 'ref'<br />
<br />
---- Backtrace ----<br />
#0 (root)/include/mysqli.php(211): osTicket->logDBError()<br />
#1 (root)/include/class.orm.php(3482): db_query()<br />
#2 (root)/include/class.orm.php(658): MySqlExecutor->execute()<br />
#3 (root)/include/class.thread.php(1961): VerySimpleModel->save()<br />
#4 (root)/include/class.thread.php(380): ThreadReferral::create()<br />
#5 (root)/include/class.ticket.php(3039): Thread->refer()<br />
#6 (root)/include/class.thread.php(1418): Ticket->systemReferral()<br />
#7 (root)/include/api.tickets.php(187): ThreadEntry::lookupByEmailHeaders()<br />
#8 (root)/include/class.mailfetch.php(84): TicketApiController->processEmail()<br />
#9 (root)/include/class.mailfetch.php(137): osTicket\Mail\Fetcher->processMessage()<br />
#10 (root)/include/class.email.php(1129): osTicket\Mail\Fetcher->processEmails()<br />
#11 (root)/include/class.mailfetch.php(254): MailBoxAccount->fetchEmails()<br />
#12 (root)/include/class.cron.php(25): osTicket\Mail\Fetcher::run()<br />
#13 (root)/include/class.cron.php(110): Cron::MailFetcher()<br />
#14 (root)/include/api.cron.php(15): Cron::run()<br />
#15 (root)/include/api.cron.php(45): CronApiController->run()<br />
#16 (root)/api/cron.php(23): LocalCronApiController::call()<br />
#17 {main}
