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Solution for sql server error in dropping database: The server principal 'DOMAIN\NAME' already exists.

If it's a SQL user, you can try:
EXEC sp_droplogin 'loginName'
or for a domain account:
EXEC sp_revokelogin 'loginName'

If they are still users on a db(s), you will have to drop the users first:
USE dbName
EXEC sp_dropuser 'username'

Naturally script out or write down all existing permissions before deleting existing users and login.

posted by Milindanath @ 1:43 AM