Getting error when adding CAS users

Hello, 

I am trying to add users mm3336 and knt22, but keep receiving the following error

  “An AJAX HTTP error occurred. HTTP Result Code: 500 Debugging information follows. Path: /batch?render=overlay&id=471&op=do StatusText: ResponseText: PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry ” for key ‘name’: INSERT INTO {users} (uid, name, pass, mail, created, status, init, data) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7); Array ( [:db_insert_placeholder_0] => 476 [:db_insert_placeholder_1] => [:db_insert_placeholder_2] => $S$D3VaSZzBv8m96mQ8Sz.lr4SAAdmKuocQl95ZP7G9OCc8ByU57/78 [:db_insert_placeholder_3] => [:db_insert_placeholder_4] => 1512679903 [:db_insert_placeholder_5] => 1 [:db_insert_placeholder_6] => [:db_insert_placeholder_7] => a:5:{s:16:”ckeditor_default”;s:1:”t”;s:20:”ckeditor_show_toggle”;s:1:”t”;s:14:”ckeditor_width”;s:4:”100%”;s:13:”ckeditor_lang”;s:2:”en”;s:18:”ckeditor_auto_lang”;s:1:”t”;} ) in drupal_write_record() (line 7383 of /mnt/www/html/yalesite582/docroot/includes/common.inc).”

I have attempted to add them in both Firefox and Chrome.