Hello I wonder if there is way to disable repeating value in SQL Server 2012. I don't want several people to have the same login. As a primary key I am using ID. Should I change primary key from ID to Login ?

Hello I wonder if there is way to disable repeating value in SQL Server 2012. I don't want several people to have the same login. As a primary key I am using ID. Should I change primary key from ID to Login ?

Declare the login to be unique.
Or, equivalently, create a unique index on login:
create unique index table_login on table(login);
First off, you should never store a password as plain text in the database. Store an encrypted string if the password must be recoverable, but better yet store a salted hash of the password.
You could enforce your rule by creating a unique constraint on the Login column.