Mailman can't completely stop you from getting duplicate messages, but it can help. One common reason people get multiple copies of a mail is that the sender has used a "group reply" function to send mail to both the list and some number of individuals. If you want to avoid getting these messages, Mailman can be set to check and see if you are in the To: or CC: lines of the message. If your address appears there, then Mailman can be told not to deliver another copy to you.
To turn this on or off using the web interface:
This can also be changed for multiple lists at the same time if you are subscribed to more than one list on the same domain. See Section 10.1 for information about changing settings globally.
To turn this on or off using the email interface:
Commands can appear in either the body or the subject of the message. (See Section 3.2 for more information about sending mail commands.)