This syntax will give you a recordset with unique attributes:
select min(directoryid) as directoryid,
group by firstname,
You can either left join this to your master table as a subquery and delete where the subquery directorid is null, or you can select this recordset into a temporary table, truncate/delete records from your existing table, and then insert the saved records back into it.
If it's not practically useful, then it's practically useless.