I have recently migrated a CiviCRM instance from Joomla to Wordpress, everything went smoothly.
The installation is using CiviCRM 5.7.2 and PHP 7.2.
We're now seeing a peculiar problem where, when users are making general use of CiviCRM, literally 100s of duplicate contacts are created of whichever user is working.
The duplicates accumulate very quickly. In about 10 minutes of experimentation with CiviMail today, I managed to create 249 duplicate contacts of myself.
In testing for this question, I created 25 contacts simply by searching for 'contacts created today' in advanced search.
Just about any action appears to create these duplicates.
When deleting them, CiviCRM will warn me that 'you are currently logged in as' one of the duplicates and therefore it cannot be deleted. Obviously my user account should not be associated with a just-created contact.
My guess is that this may be caused by something like a missing database constraint, but I'm at a bit of a loss as to where to start looking.