Eliminate Duplicate Emails On Reply All
Most mailing list have the settings so that replies go back to the individual that sent the message. And people use the Reply All command on their email reader to send a reply back to the mailing list.
This typically causes 2 messages to be sent to the original poster. One from the person that replied, and one from the mailing list.
The Eliminate Duplicate Emails From Reply All switch will stop Mail-List.com from sending the second copy.