List all users on SQL Server for auditing
Users are shown 'duplicated' simply because each database has its own users.

It may help to extend your query a little, so you can see which database each set of users is coming from:

exec sp_MSforeachdb 'select ''?'' as DBNAME, * from ?.sys.sysusers'


Note the important differences between a Login and a User :


  A Login is for Authentication.
  A Login verifies that you can access the server.
  There are two types: SQL Logins and Windows Logins.
  Logins are created at the SQL Server instance level (and stored in master).
  
  A User is for Authorization.
  Authorization decides what you can do in a database.
  A User needs to be linked to a Login.
  A User doesn't have a password - the Login decides where that is stored.
  It is quite usual to have Users from different databases linked to a single Login on the server.
  Users (and their Roles) are stored in each database, not in master (see Security &gt; Users in each database when using SSMS).