I recently ran into a problem where I needed to impersonate a user which is normally very easy, except when there a multiple users with the same name. One way to do that is to simply put in the user_id and your done (assuming you know it). Another way I just found out was by using the sys_id. I had just used Show XML to find the user's sys_id so I copied it, but when I got to impersonate I had no idea there were going to be multiple users with the same name, so I pasted the sys_id into the impersonate field and it worked like a charm. This is an edge case but nice to know never the less. Enjoy!