Error on Adding New Server Login

When I tried to add a new login to the server and after assigning DB Roles,
I am getting the error message that the role has been already added to the user. I think this username has already been in the system earlier. It is in the sysuser table. I tried to delete the roles to which this user has permissions. But still I am getting this error. I am not sure where else it is stored. How can I delete this username completely from all Databases and reassign this userid in the server.
Please advise.You're trying to modify system tables...hmmmmmmmmm

What other type of work do you do?

Look at system stored procs sp_dropuser and sp_droplogin|||I am not modifying the system tables. I am trying to add a new user.
But it is not letting me do that as this user already exists in the sysuser table. I executed sp_dropuser 'test' and sp_droplogin 'test'. But still getting the same error when I tried to add this userid and assigned DB roles(I am doing for all 4 DBs at a time).

Thanks
Radhika