Comment 1 for bug 1216960

Are you reporting this for Mailman 2.1 or 3.0? The handler you reference is the Mailman 3.0 handler, but I suspect your observation of the behavior was on a Mailman 2.1 list.

In any case, this behavior is intentional by design. Cc: recipients who are list members with their 'avoid duplicates' option set are removed from the Cc: list to keep that list from growing excessively in long threads with many 'reply-all' replies.