How to Eliminate Duplicate Emails From Reply All
If you have your mailing list configured for Replies to go back to the Author of the message, then a Reply All from a subscriber will send a copy of their reply directly to the Author, plus a copy back to the mailling list.
The mailing list will then send a second copy of that email to the Author.
Eliminate Duplicate Emails From Reply All will be grayed out
If the Have Replies Go Back to List by Default is set to Reply to List
That is because a Reply All in this case will not send a copy to the Author of the message. Neither will a Reply.