I see error messages in my PHP log like “WordPress database error Duplicate entry ‘YYYY-MM-DD’ for key ‘unique_date’ for …”

This is caused by a race condition in the code, it’s safe to ignore (it shouldn’t be labeled as an error really, but that is part of WordPress that we can’t control).

It happens when a new day starts and two visitors hit the site at nearly the same time for the first visit of the day. Both try and create a new row in the table to track the days visits, but only one of them success and the other throws this warning.

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