Hello,

On Wed, 26 Jul 2017, Danny B. wrote:

Reply-to: sender@example.com, list@lists.wikimedia.org

Pressing reply should then open a message with both addresses in the To: field. " It should be noted, that this actually technically means, that the sender will receive such reply twice - once as the direct recipient sender@example. com, once as the subscriber of the list...

There is a mailman option for this problem, too. You can find a value for new_member_options to avoid duplicates if possible.

The Readme says:

new_member_options

Each member has a set of subscription options which they can use to control how they receive messages and otherwise interact with the list. While the members can change these settings by logging into their personal options page, you might want to set the default for a number of the member options. You can do that with this variable, but see also the other categories for other member defaults you can set.

This variable presents a set of checkboxes which control the defaults for some of the member options. Conceal the member's address specifies whether or not the address is displayed in the list roster. Acknowledge the member's posting controls whether or not Mailman sends an acknowledgement to a member when they post a message to the list. Do not send a copy of a member's own post specifies whether a member posting to the list will get a copy of their own posting. Filter out duplicate messages to list members (if possible) specifies whether members who are explicitly listed as a recipient of a message (e.g. via the Cc: header) will also get a copy from Mailman.

Of course, members can always override these defaults by making changes on their membership options page.

I tried it on my site and it works fine.

In addition you have the possibility to set "duplicates off/on" for each member in the admin and the user web interface for mailman.

Cheers nosy