At All People view, open and edit a version of the duplicate, say adding a photo, after save, both record will have the updated photo displayed.

How to clean-up/remove the duplicate?

Note, if at a Confluence document, manage Watchers, typing in the person (with the duplicated record), both duplicates will appear but only one with the updated photo.

At Site Administration > User Management, can only see one reference to the person.

Apparently, the user was created by system administrator. Later, the user created himself upon the login Confluence process, and specified the same email address.

Hope the information helps.

Many thanks.
Frankey