Duplicate owner for record

One of our users has a duplicate record in the owner table.  I had not known about the existence of this entity that links records to their owner until I was troubleshooting a SQL query that was returning a wrong result.  It used the owner entity as a join and was therefore counting records owned by this user twice.

I want to delete the duplicate owner record, but am concerned that it may impact the user.  Has anyone come across this situation before and can advise? Any ideas how a user could become duplicated as an owner?

Many thanks,

Rob