This was a strange one although to be fair, the error message is self-explanatory but the root cause may not be.
This error occurred after an attempt at changing the Database Owner. The account I was changing to was already listed as the database owner in the Login Permissions but the current DB Owner was different. What probably occurred was that the DB Owner account I was changing to, had probably been used in the past and changed but the DB_OWNER user permission had not removed. Therefore when I tried to add it back in (The reason being that the legacy application didn’t function correctly with a different owner), the login/user was already (sort of a user).
To correct the issue, I simply dropped the user from the database to remove the DB_OWNER permission and then added the login as the Database Owner again:
USE DBName GO SP_DROPUSER ‘User_Name’ GO SP_CHANGEDBOWNER ‘User_Name’ GO