When multiple messages are selected in summary view and reply is invoked claws opens a reply window for each message selected. This is not very useful - if you are going to write a reply to the message anyway you can just select it separately before relying. However, opening one reply window quoting all the selected messages would be quite useful. Also the recipients should be collected from all messages according to reply/reply-all/reply-list.
*** This bug has been marked as a duplicate of bug 1450 ***