Hello,

I've made a patch to message.el to better detect if message you are
trying to cancel (or supersede) is really yours.  Personally I got
annoyed, because I use gnus-posting-styles to change different
addresses when posting to usenet.

Now there is additional check by matching message-alternative-emails
regexp (already existing variable) agains sender (grabbed from From
field) email.  I also separated common code from message-cancel-news
and message-supersede to new function message-is-yours-p, because same
code was duplicated.

Patch is here: