There are two possible behaviours:
 
1. SET treats the two addresses as if they were different people, and you
   may need to have both in the list so that people can post.
 
2. SET treats  the two addresses as  if they were the  same person (which
   they are), and  you do NOT need  two addresses to allow  the poster to
   send mail from the other address.
 
Anything else is a bug that should be reported.
 
  Eric