Description
I use wp cron event run --due-now as my production cron. Every 5 minutes. But it's a heavy cron. Sometimes it takes more then 5 minutes. When that happens some events get executed twice. Mostly sending emails.
I would suggest a --no-doubles flag or something like that to prevent the cron from doing hooks twice.
When the flag is set, wp-cli should check for the doing_cron transient like core does. And set it if it's starting. Like core does.
Why is it doing things twice
At the start of a cron all tasks are gathered with _get_cron_array, events get deleted once the start. And the remaing cron array is still executed. And still in the DB list So after 5 minutes the new cron get also does a _get_cron_array and will get all events that the first cron also is still handling (and deleting as it goes).