update_dupes is not currently a feature but was a thought of how I might implement your request in the future.
This might work, but you'd want/have to clear out all existing duplicates on your list:
Use the allow_dupes option.
Copy signup-xtra.php from ./xtra to the main Listmail folder. Modify the file to contain the following code, replacing the list number with your desired list and user1 with your desired custom field:
<?php$mylist = '1';if($list==$mylist){ // update existing entries if exist mysql_query("update lm_users set user1 = '".addslashes($user1)."' where list = '$mylist' and uid <> '$uid';"); if(mysql_affected_rows()>0){ // delete the new user only if already existed mysql_query("delete from lm_users where uid = '$uid';"); }}?>
Regards