How did happened? Duplicate contact

Bumping an old issue as I am seeing similar behaviour all of a sudden in Mautic 4.4.1

I have two new contacts created. There lead ID are consecutive: 483 and 484.

For lead id 483 this is the audit log:


In the history of this user I am not seeing any tracking, however they are getting an email sent and changing into different segments and campaigns.

For lead id 484 this is the audit log:


For this user I am seeing a full history of all pages being tracked as well as emails being send.

The emails being sent are from within a campaign and they are market type “marketing”.

The field for email is publicly updatable and is also a unique identifier.
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The user is being created via the Mautic tracking script and/or via forms in both cases.

@joeyk or anyone else looking for some interesting insights here.
My initial hunch was that a field was being overwritten on a specific user, but I would expect to see the values of email change in the audit log if this was the case, and then the fact that the lead_ids are consecutive also raises a small alarm.