[Mailman-Users] Email Aliases for subscribers?

[Lewis Lau]

Dear friends,

Let say I have a subscriber with email address "wtlewis at somewhere.com" on a 
member-only list, then he also have an email alias 
"lewis.lau at somewhere.com" refer to the same email account. Is that any way 
to tell the mailman these two email addresses are actually the same person, 
so that this member can post message onto the list no matter the message is 
from "wtlewi at somewhere.com" or from "lewis.lau at somewhere.com"?

I've tried to add the email alias onto the list as well, but this cause a 
problem that the subscriber will receive duplicate message.

Regards,
Lewis