Since case in the username (the left of the '@' in an address) is
 significant on some systems, LISTSERV now considers them distinct
 mailing addresses.  The "del" command will match both however.  So,
 the answer to you question is that it's a feature.  The best way to
 handle the case where someone has signed on twice is to "quiet del"
 the address, which will remove both, then "quiet add" the one you
 would like to be used on the list.
 
 -jj