The Reply button has posted the same comment twice when I clicked it twice (over a slow connection).
It happened today [[ https://www.mediawiki.org/w/index.php?title=Topic:Sf46ckup98xhelax&topic_postId=sf47l09ztv7njy9o&topic_revId=sf47l09ztv7njy9o&action=single-view | here ]] and [[ https://www.mediawiki.org/w/index.php?title=Topic:Sf46ckup98xhelax&topic_postId=sf47l09ztv7njy9o&topic_revId=sf47qdqrta7nrgu8&action=single-view | here ]] while posting to [[ https://www.mediawiki.org/wiki/Topic:Sf46ckup98xhelax | this thread ]].
The action should be idempotent - the server should check that pressing the Reply button many times for the same post are only processed once. A simple way to do this is to attach a hash value to each new post when the user starts it, and sending it with the Submit action.