Since this is a mailing-list, I use reply to mailing list. It also happens when I simply use reply to. The problem does *not* occur when I originate an email, however (no replying).
As a test, I simply clicked reply in order to answer this post. Amazingly, the To: field is correct. In the past, I have seen that sometimes *To* causes the mail to be sent to both the list and to the originator of the post (which leads to a lot of attempts at unnecessarily taking conversations off-list and private). This has happened to me a few times because I did not pay attention to the To field when replying.
So, here goes, sending a response with just Reply, not Reply to Mailing-List.