Product:
Cognos Controller 10.4.2
Microsoft Windows server 2012 R2
Microsoft SQL Server 2016

Problem:
After restore of one old Cognos Controller database backup, you get this error when you try to create a SQL account for the database.

Error:
The server principal ‘fastnet’ already exists

Solution:
Go into Microsoft SQL Server Management Studio as Administrator
Mark your newly restored controller database
Expand the Tables and check the schema name (mostly it is fastnet)
Click on “New Query”
Past in this code (edit the name and update the ‘ to be correct symbol)
EXEC sp_change_users_login ‘Auto_Fix’, ‘fastnet’
Click on Execute.

The row for user ‘fastnet’ will be fixed by updating its login link to a login already in existence.
The number of orphaned users fixed by updating users was 1.
The number of orphaned users fixed by adding new logins and then updating users was 0.

This should create and update the SQL server with the account used for the controller database.

More information:
https://sqlserverbuilds.blogspot.com/
https://www.crestwood.com/2017/05/24/error-creating-new-sql-user-server-principal-already-exists/