Error 15023: User already exists in current database.

The problem is that the user in the database is an "orphan". This means that there is no login id or password associated with the user. This is true even if there is a login id that matches the user, since there is a GUID (called a SID in Microsoft-speak) that has to match as well.

This used to be a pain to fix, but currently there is a stored procedure that does the heavy lifting.

All of these instructions should be done as a database admin, with the restored database selected.

First, make sure that this is the problem. This will lists the orphaned users:

EXEC sp_change_users_login 'Report'

If you already have a login id and password for this user, fix it by doing:

EXEC sp_change_users_login 'Auto_Fix', 'user'

If you want to create a new login id and password for this user, fix it by doing:

EXEC sp_change_users_login 'Auto_Fix', 'user', 'login', 'password'

Here are some stored procedures that could possibly automate the process

Stored Procedure 1:
/*Following Stored Procedure will fix all the Orphan users in database
by mapping them to username already exist for user on server.
This SP is required when user has been created at server level but does
not show up as user in database.*/
CREATE PROCEDURE dbo.spDBA_FixOrphanUsers
AS
DECLARE @username VARCHAR(25)
DECLARE GetOrphanUsers CURSOR
FOR
SELECT UserName = name
FROM sysusers
WHERE issqluser = 1
AND (sid IS NOT NULL
AND sid <> 0x0)
AND SUSER_SNAME(sid) IS NULL
ORDER BY name
OPEN GetOrphanUsers
FETCH NEXT
FROM GetOrphanUsers
INTO @username
WHILE @@FETCH_STATUS = 0
BEGIN
IF @username='dbo'
EXEC sp_changedbowner 'sa'
ELSE
EXEC sp_change_users_login 'update_one', @username, @username
FETCH NEXT
FROM GetOrphanUsers
INTO @username
END
CLOSE GetOrphanUsers
DEALLOCATE GetOrphanUsers
GO

Stored Procedure 2:
/*Following Stored Procedure will fix all the Orphan users in database
by creating the server level user selecting same password as username.
Make sure that you change all the password once users are created*/
CREATE PROCEDURE dbo.spDBA_FixOrphanUsersPassWord
AS
DECLARE @username VARCHAR(25)
DECLARE @password VARCHAR(25)
DECLARE GetOrphanUsers CURSOR
FOR
SELECT UserName = name
FROM sysusers
WHERE issqluser = 1
AND (sid IS NOT NULL
AND sid <> 0x0)
AND SUSER_SNAME(sid) IS NULL
ORDER BY name
OPEN GetOrphanUsers
FETCH NEXT
FROM GetOrphanUsers
INTO @username
SET @password = @username
WHILE @@FETCH_STATUS = 0
BEGIN
IF @username='dbo'
EXEC sp_changedbowner 'sa'
ELSE
EXEC sp_change_users_login 'Auto_Fix', @username, NULL, @password
FETCH NEXT
FROM GetOrphanUsers
INTO @username
END
CLOSE GetOrphanUsers
DEALLOCATE GetOrphanUsers
GO

Stored Procedure 3:
----Following Stored Procedure will drop all the Orphan users in database.
----If you need any of those users, you can create them again.
CREATE PROCEDURE dbo.spDBA_DropOrphanUsers
AS
DECLARE @username VARCHAR(25)
DECLARE GetOrphanUsers CURSOR
FOR
SELECT UserName = name
FROM sysusers
WHERE issqluser = 1
AND (sid IS NOT NULL
AND sid <> 0x0)
AND SUSER_SNAME(sid) IS NULL
ORDER BY name
OPEN GetOrphanUsers
FETCH NEXT
FROM GetOrphanUsers
INTO @username
WHILE @@FETCH_STATUS = 0
BEGIN
IF @username='dbo'
EXEC sp_changedbowner 'sa'
ELSE
EXEC sp_dropuser @username
FETCH NEXT
FROM GetOrphanUsers
INTO @username
END
CLOSE GetOrphanUsers
DEALLOCATE GetOrphanUsers
GO