Redundant mails when using multiple recipient lists

[seirerman]

One of our customers sends newsletters to multiple tt_address recipient lists. Some of the subscribers are on several of those lists, which means that they get the same newsletter multiple times.

We created a composer patch (using cweagans/composer-patches) so that direct_mail checks the combined list of recipients and removes redundant records before sending the mails.

Feel free to include it in direct_mail!

direct_mail-prevent-duplicate-mails.patch