I found another method to fix this problem. Using phpMyAdmin I entered this SQL query:
select bad_rows.*
from wp_users as bad_rows
inner join (
select user_login, MIN(id) as min_id
from wp_users
group by user_login
having count(*) > 1
) as good_rows on good_rows.user_login = bad_rows.user_login
and good_rows.min_id <> bad_rows.id;
This gave me a list of duplicates in which I could delete all of the records shown to remove the dupes. Thanks again for your help Trent!