Thanks for your post. It sounds like your script that you are specifying in Multiscript is attempting to add a new user object that already exists in the destination SQL Server database. This message is coming from the SQL Server itself as a result of the script.
To get the script working properly, you would need to manually edit the script to remove the entry that is making changes to this particular user object. Also, since this is the master database, this is a system DB, so you will need to be careful about what changes you are rolling into it.
Red Gate Software Ltd.