If you edit the existing spam post, post is validated against all registered anti spam providers.
I did a post on a MonoX installation using Defensio and it was marked as spam, but when I then edited the post and submitted again, it wasn't spam anymore (had just added two more rows of text to the end of the post)
So I wonder if MonoX is consulting the spam engine again at post edit or maybe there is a bug there?
Is there a chance that you received two messages because you are Administrator and Owner of the post ? This was the case with version <4.7 as there was a bug related to notification send out, which version are you using ?
moreover I got two e-mail notifications WITH THE SAME TEXT (the version of the post I did 2nd, with the added row of text)