mysql database update possible timing issue
The issue was that since the queue run for 1 minute, there were cases that one cronjob was not finished yet, and another was starting. So they were processing the same tasks from the database.

The solution is to run cronjob using flock(), thus making sure that only 1 is run:

* * * * * /usr/bin/flock -n /tmp/fcj.lockfile /usr/local/bin/frequent_cron_job