Do you mean you add both the user and mailing list as recipients? Yes it will be duplicated because the mailing list is not expanded to all members while spam scanning, this helps save system resource for spam scanning as it's only 1 message scanned, otherwise if the mailing list contains 100 members, there will be 100 messages scanned, it's a waste of system resource.

I have it. On the one hand i mailist created with the subscribers of the list. These subscribers receive email sent to the list but cannot send in the name of the list. For this, we must add them to memberofgroup and that is where duplicate e-mails. If you deleted the subscriber of the mailist and left him alone in memberofgroup, corrects the problem. The issue is that in this way you lose control of who are members of the mailing lists.
The question is, there is a way to centralize the subscribers of the lists to know at a given moment who are the members of list without having to go on a user-by-user? I do not know whether I have explained well..
Thank you very much

And that is what we are doing. The command we launched it with the following options:
"python maillist_admin.py create mylist@mydomain.es only_subscriber_can_post=no disable_archive=yes"
And add users to the list:
"python maillist_admin.py add_subscribers mylist@mydomain.es user1@mydomain.es user2@...."

Thank you for your help

If you do not add the memberofgroup attribute to the user, unable to send on behalf of maillist

For if it helps, the error that returns us when trying to send on behalf of a maillist is as follows "Recipient address rejected: Sender is not same as SMTP authenticate username"
How to e-mail client we are using "Thunderbird"

Thank you and a greeting,