SQL Server User Mapping Error 15023
<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>