title: SQL Server User Mapping Error 15023
body: <p>I try to map my other DB to a user by going to <br/>Security > Logins > right click someuser > Properties > User Mapping > Select DB > set as db_owner and then ok, but I keep on getting an error saying </p>
  
  <blockquote>
    <p>User, group, or role 'someuser' already exists in the current database. (Microsoft SQL Server, Error: 15023)</p>
  </blockquote>
  
  <p>What is causing the error, and how do I map that user to the database? </p>