I created a sql account and wanted to give it dbo access to a particular database.  The settings didnt set for dbo when I created the account, then when I go back and try to adjust the user settings, I get a user already exists error.  The user does already exist, but I just want to update it, not recreate it.  Eventually I found out from reading here and here that my login was orphaned and needed to be mapped to a sql account.  I got the list of orphaned logins with this:

EXEC sp_change_users_login 'Report'

and I fixed it with this

EXEC sp_change_users_login 'Auto_Fix', 'myuser', NULL, 'mypassword'

The only remaining hurdle was I had to set check_policy = off.  I tried some variations of set check_policy = off, all of which gave me errors, but one worked apparently, cuz I was finally about to exec that statement.